It sounds like they are being very vague about the measurements and centiles. "75th to 91st centile" is a huge band, where "50th centile" sounds very precise.
50th is average, but that will be for children as a whole - she can still be the tallest in her class, if she happens to be in a class of shorties!
And being on a different centile for height and weight doesn't mean that she's overweight (or underweight), just a little more solidly built than average.
It sounds like it's worth asking the school nurse what the actual figures and centiles were, to get proper results.