I think it's very common to see more differentiation in reading than math. I think schools recognize that literacy is partly dependent on developmental readiness, and therefore 'clicks' at different ages, resulting in a big range in ability in an age-levelled classroom. However, they often labour under the illusion that early math ability is about teaching, rote memory, practice and drill, and therefore a lockstep approach is all that's needed. Or that advanced kids can always benefit from more drill of the basics. There's also a fear that if kids get a year or two ahead in math, they'll "run out" of elementary school curriculum, and then what will they do in 6th grade?
That's great that meemee has seen a real willingness to differentiate in math. My limited experience with our public elementary school suggests that reading differentiation is much more likely than math.
I don't want to hi-jack the reading with math talk, so a quick comment.
I find that many elementary teachers aren't mathematical thinkers. They see math as rote learning because they think it in verbal language and not visual spatial math language. Thus the "Kids get the right answers without understanding" message. I had the amazing opportunity as a kid to be in a mathematics club (all kids doing the national mathematics competition) and we were pulled out for our own class weekly, and got to really play with and be creative with math. I still love math, and will do math puzzles to relax. I love calculus! I have elementary teachers telling me "All kids hate math" if I try to broach the subject of making math more interesting for my daughter. I couldn't find anything like a math club for my 2E son, who does love math (and sometimes the teachers act like this is pathological) and in his case Asperger's and verbally explaining method (the way it's done in typical elementary education) were a bad match. Finally, he got a teacher last year who allows drawing and pattern mapping (with numbers) the thought process, rather than explaining or repeating her method. It made a big difference and we got a lot more help with giving him math at his level (he's in grade 5 and at about grade 8 level for math), but at lower volume as his speed is behind. He gets to test with grade level material, and the higher level stuff is enrichment to keep up interest.
As for the reading, I think a higher reading level is much more apparent to the average classroom teacher. Really, she/he could simply get the child to read aloud from a boook at the suspected higher reading level, and ask a few comprehension questions. It should be pretty apparent if they are ready for more.