Well, there are two reasons, I believe. One is the fact that the longer in advance you buy it, the cheaper it is and the second is that peak vs. off-peak times

Well, there are two reasons, I believe. One is the fact that the longer in advance you buy it, the cheaper it is and the second is that peak vs. off-peak times matter a lot. If you buy one month in advance and at non-peak times (e.g. 12:00 on a Wednesday or similar), it will be around £30-40 probably.