The reason for that is that the bank limit for the machine has been set at £30. You can change - on a real machine as well as an emulated one - the amount that the player can have in the bank at any time.
Most machines in pubs I've played have a limit of about £60, when you get that in the bank, the machine will automatically pay out any excess, leaving £60 in the bank. Some machines as well [often £5 JP] will have a bank limit of zero, meaning they pay out any win as soon as it is collected. Generally for a machine to be set like that would suggest that the hopper has been floated with about £25, rather than the recommended £125...or is it £75 for a £5 machine?
Anyway, £30 for a £8 JP machine is about right I'd say
Ben
Hopefully recovering from years of compulsive gambling and wanting to be gamble free forever.
Recommended reading -
http://www.gamblersaloud.com/ (yes, I bought the book, very happy with it!)