On the face of it, most people think of rifle and pistol ammunition as all being the same. This is much like ignorance of any subject; you do not have a frame of reference with which to determine either the similarities or differences between objects in a certain field. To make this point clear — this notion is immediately apparent in the field of coffee and tea. Isn't all coffee the same? It all tastes like coffee, after all. Well, no, it isn't all the same. Neither is all tea. Neither are bullets.
That isn't to say that there aren't shared characteristics between bullets (or coffee or tea). Some may be based on similar designs and use the same diameter bullet. They may use the same kind of powder and primer. Yet, the end result is quite different. Much like coffee and tea, change a couple of the variables and you have something that is different, perhaps subtle, but distinct.
Even for the seasoned shooter, caliber nomenclature (by this I mean, what is actually marked on the box), on the surface, does not seem to have any rhyme or reason associated with why X cartridge is really called X. This was not always so.
In the time of the first self contained cartridges (1860's or so), things were very simple. You were given the caliber, the length of the cartridge and the amount of black powder to be used. So, a typical cartridge of this period would be listed as (.)45-210-70 (the 210 means 2.1 inches in length). Eventually, this nomenclature was somewhat shortened to just the caliber (.45) and the powder charge (70 grains) which ended up, in this example, giving us the .45-70 Government.
When smokeless powder came about in the early 1900's, the naming conventions mainly stayed the same. For instance, the .30-30 Winchester, a .30 caliber bullet with a powder charge of 30 grains of black powder, was the first to make the change to smokeless powder. In so doing, it lost the meaning of its name — it does not take nearly 30 grains of any powder today (think more like 10) to make the cartridge work. But we still call it the "30-30" and life goes on.
Today, things are much more difficult to discern. No longer satisfied with caliber, case length and powder charge, the cartridges of today in their designation are much more of a mystery. Other than the metric system to designate cartridges, it is really just a matter of, more often than not, "what can we call this one?"
For instance, there are only 6-7 different (this is not considering bullets that are custom made; nor is it taking into account pistol bullets) diameters of rifle bullets readily available. One of the most popular bullet sizes available today is the .224" caliber bullet. This bullet size is used in the following cartridges:
As you can see, only one of these has the "correct" nomenclature for the actual diameter of the bullet; yet, they all shoot the same bullet. (As an aside, all bullets are oversized one thousandth of an inch over the bore size in which they are to be used. The reason the bullet is manufactured this way is so that the bullet engages the rifling and has spin imparted to it, thus making it more accurate.) Why the discrepancy? In a word:
How many times can something be made whiter and brighter? About as often as is needed, it seems. This isn't to say that all of these are creations just to fill some new found "need". Taking the above example, all of the above are just as distinct as Jamaican Blue Mountain versus a cup of Folgers. They each use the same diameter bullet (they are all coffee), and yet, they all perform a different function (they all have a different taste). In this case, both the .220 Swift and .22-250 Remington are long range, high speed varmint hunting rounds. Both can be hard on barrels- too hot to handle, in a sense; high pressure and high velocity do not make for long barrel life. Whereas the .223 Remington has roughly half the amount of speed and gives up some range, but is in demand more than either of the former. All of them have different brass casings. Just a variable here, a variable there and you end up with something that has different working pressures, effective ranges and velocities. They all serve a purpose; it is just a matter of knowing which one to pick for that purpose.
However, it is still very much a convention of marketing. Who else (than a company) would name something "Zipper" or "Bee"? Some of this is probably an attempt to make the cartridge stand out, which is, of course, quite along the lines of the text from a Marketing 101 textbook. The differences between them lay not only in the names, but in their respective abilities. The names just don't tell you what their abilities (or even what size bullet is being used) are; which is, of course, the problem.
In the end, the best way to find out what the caliber nomenclature means is to ask your local sporting goods shopkeep to get the scoop. Because if you just go by the box, you will be muddled quite badly.