Answer 1

There can be many reasons why. Usually it has to do with the browser's user agent stylesheet and how it fits in with yours. Only taking a look at the code first hand could we know for sure. Inspecting the affected area in Chrome often helps identify the problem.

Otherwise, generically speaking...

Chrome uses a vendor prefix for its scrollbar properties ("-webkit-scrollbar").

More on that here:
https://css-tricks.com/almanac/properties/s/scrollbar/

This may or may not be relevant to your problem at all, but again, we can't know what the problem is for sure without looking at the actual code.