Well there are a couple of ways to shave and I am not sure you would get near 98% by doing monthly tests. Nothing keeps you from doing monthly test, six months in a row on one account.
One way programs shave and this is the most blatant, stupid and outdated method is to just “not track” first time players. This has the greatest potential to get exposed.
The other more subtle way is to skim off the cream which is almost undetectable for most affiliates but I am sure is employed a lot. Also for the average affiliate this can be the most damaging. If a player who plays for ten months gets dropped after month 1 that is costly!