The GSM8k problems are not "adding numbers." They are word problems, e.g. "Katy makes coffee using teaspoons of sugar and cups of water in the ratio of 7:13. If she used a total of 120 teaspoons of sugar and cups of water, calculate the number of teaspoonfuls of sugar she used."

They are the kind of problems that, if your teacher was anything like mine, were usually skipped in order to keep the slower students from bogging down the class as a whole. 0.996 (MiMo-V2.5) is substantially better than what the vast majority of humans would do.

If you limited the question to adding arbitrary pairs of numbers of reasonable size, I imagine quite a few models could get to 1.000.