In many cases, we get numbers by collecting a few thousand data points and applying analysis to them. The aforementioned law of large numbers plays a part in it. Sometimes trends emerge easily from the data, sometimes you need to apply a curve fitting algorithm or other analysis technique.
A simple example of this is found in my analysis of warrior threat values (http://www.tankspot.com/forums/f200/...s-warrior.html). In this case, I wrote an addon specifically to help me pull out the numbers. Since I was able to query the game client directly for total accumulated threat values there was no need need to collect thousands of data points, merely dozens.
In some cases you can pick apart the game data files and find numbers that are hardcoded into them (this is an example of data mining.)
The formula for mitigation from armour is taken directly from the game's user interface files. The 75% hard cutoff for mitigation is also explicitly shown in that. (See http://www.tankspot.com/forums/f63/4...itigation.html for specifics.)
And sometimes it's simple observation. In the specific case of defense, for example, you can go buy a crappy grey bracer from an armour vendor and get it enchanted with deflection, +5 defense rating, which should gain you +1 defense skill at level 80 (4.91850 defense rating = 1 defense skill at level 80.) With no gear on, note your miss, dodge, parry, block values. Equip the crappy bracer that increases your defense skill by one point to 401, note that each of miss, dodge, parry, block chances have gone up by 0.04. (This is also one of those very rare cases where Blizzard has confirmed the numbers by the simple expedient of discussing mechanics in terms of them with us)