Those numbers are a pretty good baseline figure, but I always figure consider they can vary as much as 5% in either direction usually the wrong way when trying to use them to judge a slow. They are also not the percentage the mob is slowed but the percentage yer slow is mitigated as explained by Telosk & Zethrir.
Some simple reasons its not 100% accurate are skill check things like flurry and double/triple/quad attack skill level of example mob which play a role in how manys swings you see hit or miss but are not effected by attack speed, for example the same mitigation from a mob who only double attacks is going to show a larger percentage slow then a mob who quads, etc.
Here is what I use and where I compiled my info from:
Fully>Mostly>Partially>Slightly
http://forums.interealms.com/shaman/sho ... adid=15649
Turgurs 75% slow
mostly = 75 = 56.25%
partially = 50 = 37.5%
slightly = 25 = 18.75%
Grummus 50% slow
mostly = 75 = 37.5%
partially = 50 = 25%
slightly = 25 = 12.5%