Another difference between British and American English(es)

When living in the UK, I was corrected several times when I uttered the word "math" in public.

"It mathsss," they would say, emphasizing the final s.

"Why?" was what I asked after the second or third correction; it was obvious that the first correction wasn't done by a loon, after all, and that I might well be in the wrong.

"Well, it's because it's a contraction of mathematics. After all, you wouldn't say mathematic would you?"

I had to concede the point: the term mathematic was not in my lexicon, not in their lexicons, and therefore the concept of  

mathematics maths

made a sort of logical sense, and I questioned it no further. (I did, however, switch back to merely saying "math" when I moved to the US; fitting in linguistically was another lesson that I learned from this - and many other - language lessons learned in the UK.)

Recently, a lot of commentators on the left made fun of GOP Presidential Candidate, Mitt Romney, for his repeated use of the word sport instead of sports. I know of two times when he said sport as part of a public speech. It was used - among other mannerisms - to paint a picture of how he is "out of touch" with common Americans. This post is not about whether this is true, but to ask the question:

Why do Americans say sports but not maths, and (conversely) why do the British say sport and not math?

The only times that I recall hearing an American say sport were by older - usually White - men, using it in the informal way that was (apparently) more typical during their youth. For example, sentences like, "He was a good sport," or, "Hey, sport, let's get going," or even, "It was said in sport". These are rather specialized usages, though, and definitely not the meaning used by Mr. Romney, which was what most Americans would call sports.

Looking at Wikipedia's entry, it looks like some British people do make a distinction between sport and sports; the former used for talking about the overall concept and the latter reserved for explicitly talking about multiple different sporting activities. However, looking at the ngram of sport vs. sports in British English shows that the latter meaning is a recent one, gaining parity around 1976-1978, and is only slightly more dominant at present:

Meanwhile, the ngram for American English shows the shift between sport and sports occurring back in the 1940s, and sport occurring only slightly more than half as often as sports.

This is likely why the use of sport sounds so strange to American ears. However, this still doesn't answer the main question that I had, which was, why the Brits still use sport but not use math in their lexicon?

I guess that I've just rediscovered the small point that language is not always consistent.