Some slots machines payout will change from time to time. Slot machines can contain what is referred to as variant programming. Variant programming means that the slot machine can adjust it's payout a few percentage points in either direction. So a machine that is programmed to payout 95% could payout 92% or 97%. This accounts for those times that the same slot machine acts like a completely different slot machine. and badly other times.
Slot Tip:
Casinos don't hide or move their best slot machines around because the are affraid of someone finding them. The casino want these machines to
be played because the more they are played the more other machines are played and the more money a casino can make.