Can offers to international students be rescinded if grades received following the application slightly lower their GPA?
As someone with some experience on the other end of math graduate admissions, let me preface this by saying I really doubt that anyone cares; a program like UCLA looks at a lot more than grades (especially for foreign students).  At this point it would be really disruptive for them to drop a student from the program (it's not like there are other students of the same caliber waiting around to be admitted), so they are not looking for reasons to do so.  This is probably hard to imagine after many years of people telling your grades are the most important thing, but for math grad school, they are really actually not.  They are a proxy for the ability to do good research in math, and the people running the program know they are a bad proxy and take them with a grain of salt.  

So, the important thing to do here is calm down.  Imagine this from the perspective of the department; all the people making these decisions have a lot of other things to worry about, you are one of many grad students they admitted this year, and these grades are a very small part of why. Your grades are not something they want to expend energy paying attention to.   Even if your offer is conditional, I think this condition would only be used in really dramatic cases; I've never heard of this happening.

Look carefully at your offer letter.  Either it will say the offer is contingent on your grades this semester, or it won't.  (Incidentally, this info should have been in your post; answering questions well on this site is hard enough without people withholding obviously important information).  If it doesn't, and they haven't asked about your grades, then drop the subject from your mind and never speak of it again.  If it does, I would probably still not do anything unless they ask.  If they really want that information they'll ask for it.  If they ask, just write back saying "I got an 8/10 in X and 9/10 in Y.  Looking forward to meeting you in August." Don't try to explain why your grades are not what you expected. Just act as if everything is normal, and I'm sure things will be fine.