i always thought it was because of the amount you end up drinking through each methond:
1. beer before liquor
the beer, you don't feel the effects/buzz as fast so you drink 3, 4, 5 and then start hitting the liquor, say...3-5 shots, because you don't think you're buzzed or not feeling it. so now you've got an accumulated (up to) 5 beers and 5 shots.
2. liquor before beer
you take your 3-5 shots first and almost immediately so you think "hmm, i'm getting kinda dizzy/buzzed already" and then take maybe 1 or 2 more beers to maintain it. in this scenario your total amount taken in is 5 shots, 2 beers.
(numbers theoretical, vary with how much it takes you to start feeling the effects).
hmmm, might have to experiement (in the name of science of course :beer: )