Welcome to Flyertalk and welcome to the BA forum.
There are a couple of points that could explain this. Firstly,exchange rate differences (usually a small amount) and secondly if the carrier surcharge has altered, which happens very frequently. It's also possible that they are charging you - correctly - 2 lots of £30 for each main leg. Usually speaking these tickets are best rebooked to a future date since the refund is so pathetic. You can change some details on the rebooked trip, and you have (generally) a year to play with there.