My daughter & I noticed the same thing yesterday. I paid closer attention today to the numbers.
We ran 5 route PQ's. Three are maxed out at 500 caches which we were expecting and the preview PQ agrees with the number.
For the other 2, they're off by about 100 caches each. For mine, the preview shows 463 (which is about what I usually get in th PQ), but the PQ returns only 363. For my daughter, the preview shows 499, but the PQ returns only 405.