Welcome!
With the information you've provided, it's hard to say what the reason for the difference might be. It could range from an off day all the way to an actual difference in his math and reading skills. I will note that these scores are percentiles, not percents, so the 64th %ile is quite comfortably average, demonstrating grade-level skills. It may be simply that he hasn't been instructed in the above-grade-level skills in math that would be necessary to score in the top percentiles.
It sounds like he will have exposure to higher-level instruction or enrichment next year anyway, so you'll have opportunity to see if his math scores increase with that level of instruction.
