Ib background check really need help!

#1

I have accepted a full time offer from one of the largest america investment banks, but i am now quite worry about the background check, i put down my alevel grades on application form stage according to my memory , and it probably not very accurate (maybe one or two courses different, probably a grade higher), as i took them 7 years ago, everything else on my application form is true(under, post grad history, work experience)

I wonder is this going to be a huge problem that can cause my offer withdraw? do I have any chance to explain if the grades in my memory are really not match to the reality? as i really didnt mean to lie on my application. thank you for helping me.