In our district, there is money available to make accommodations for an ASD diagnosis (they can call in specialists, hire an aide, provide social skills classes, etc), but not necessarily for more minor or different diagnoses (e.g. SPD or giftedness).

If the school believes a student needs X, Y, and Z accommodations, but the only way to get funding for them is to claim ASD, then that's what they're likely to do, whether or not that's the true cause of difficulties.