Probably absolutely nothing to worry about, several possible explanations and some have already been advanced
I do, however always suggest that people who plan to invest many thousands of pounds will find it worth investing a few less thousands of pounds in advance by going for Class 1 Medical before undertaking expenditure, study, etc
That way you will find out if there is a problem, which there probably isn't,and it is likely you may get the latest CAA-type of advice on minimizing this kind of experience
Might be as well not to explain the problem or seek guidance until after the MO tells you your hearing is of the required standard, don't want to twitch them in advance and get em looking for probs - the test is very straightforward - headset in a soundproof box with tones across the whole sound spectrum - several of each, becoming louder and you have to press a button when you hear and release when the tone becomes inaudible - something like that - go for the Class 1 first would be my suggestion