Crysta is set up so you can only buy blocks on money, so if you pay £12.95 you have to get £13.00 ( i think?), this means you paid 5p too much (you still have that 5p balance but you can't get it back either).

The next month you owe £12.90 (-5p from last month) but you can still only buy blocks so again you buy £13.00 and now you have 10p excess, and on and on. You can eventually pay less when it's saved up enough but most people forget to do this or they quit before it happens and in essence it's a sneaky way to gouge a little extra money out of a lot of people.