LOLserver: Match game only changes your match statistics (vs, hack, amateur, pro, master, tour master, legend and tour Legend) not your average.
Excuse me, not true, as explained here by Admin.
As a part of anti-sandbagging measures, match wins against higher tiers would reduce the average by several tenths until reaching Legend.
I made Legend in 2012 with two wins, dropping 0.7 off my average each.
Today, the influence is less regular, seemingly sometimes yes, sometimes no.