Main Nav Bar CSS on Hover Inconsistencies

I installed the EduBlogs theme this morning and edited the colours of the nav bar but I've got an inconsistency with the sizes of the 'children' levels on the main nav bar.

Main nav is like this:
[First Menu] [Second Menu] [Third Menu] --- etc

In the drop-down of [First Menu] the children are not the same height as 'children' in [Second Menu] etc.,

But when I hover over the 'children' in [First Menu] they expand to the same as all the others.

Not a major issue but it's driving me batty!

Any thoughts on which bit of CSS I need to tweak?

Thanks.

Martin