Yes Sir, grit ratings are one of the most confusing things about sharpening. Part of this is because there are multiple rating systems that all use similar numbers, but the numbers mean different things in terms of coarseness. Examples are JIS, FEPA, and ANSI. Look at the grand unified grit chart in the stickies section and you'll find a giant conversion table so you can compare them.
But the real part that's confusing is abrasive materials science. Different substances cut differently. For example, waterstones are generally make from aluminum oxide, which is friable. That funny word means that the abrasive breaks down as you use it. The points of the abrasive particles break off and expose more fresh abrasive below them. But... waterstones are used with water, so a slurry is formed by these broken down particles. This tends to produce a finer finish than you would expect, because the slurry, lubricated by water, contains finer abrasives (broken down) then the stone itself. The combination of these factors makes the waterstone finish different than any other. It cuts kinda fast, yet produces a finish that's finer than you might expect.
Contrast this with diamond abrasives. Diamonds do not break down. There is no slurry. Even more importantly, diamonds have a very pointy shape which makes them "sharp". They are so hard and so sharp (compared to other abrasives), that they dig into steel much deeper than you might expect. So a "320 grit" diamond plate will tend to produce a more coarse finish than an aluminum oxide 320 grit stone or belt.
What abrasive to use for what task becomes a matter of experience. A lot of people with expensive, shiny, and "finely made" blades tend to be very scared of "destroying the blade" with an abrasive that's too coarse. In my own experience, I often use abrasives in the 60 to 120 micron range (about 320 grit to 100 grit). Blades that are very dull need a lot of abrasion. Blades that are nearly sharp don't need very much. It's very much a matter of the right tool for the right job.
I'll stop here and hope that this has helped some.
Brian.