Never knew this before last night.
If you are playing games like Gates of Olympus, Wild wild riches, sweet Bonanza etc where there is an option to increase the bet to improve the chances of a feature, this part of the bet doesn't count towards the max win.
Gates of Olympus went off its tits again last night, and stopped at £4000.
Was playing £1 stake, but 20p of that was the feature button - if that had been off and I was playing a straight £1 stake, the win would have been £5k.
Think that's a bit cheeky from the game providers.