I was chatting with an engineer who explained how it works....
Apparently the random number generator randomly sequences something like 100,000 numbers, so you may have something like this....
005102
013412
000199
087142
041023
etc...
Each of these numbers represents a particular pattern of the reels and therefore a particular payout.
After playing all 100,000 "games", the payout will have been 94/95% of the take. (Then a new sequence is generated).
Because the sequence is random, there is every chance that two £500 payments will be very close together.
This is the only way in which the result of your game is totally random, yet is also guaranteed to payout 94/95% over the period of a sequence.