The reason is that we don't know what the next rate will be. So if you bought in April you are guaranteed the 7.12% and then 9.6% return, versus buying in May

The reason is that we don't know what the next rate will be. So if you bought in April you are guaranteed the 7.12% and then 9.6% return, versus buying in May you will get 9.6% for 6 months and then an unknown rate after that. It could be more, it could be less - most people are expecting it to be lower.