They are not bothered. It may mean that a certain night gets some people in at £3 and some at £4. The just tell the selling outlets the time to put the price up as they redliver the next batch. Its the same for other clubs here when you buy tickets in advance. Many are valid for 6 months. If you buy in advance youve made a profit but theyve had your money anyway. They will make more from people keeping/losing chips than they lose from a price rise.