90 < _ < 100 means that the number could be anyone of these numbers:
91, 92, 93, 94, 95, 96, 97, 98, 99
640 < _ < 650 means that the number could be any of these numbers:
641, 642, 643, 644, 645, 646, 647, 648, 649
Note that it is ANY of those numbers, NOT all of those numbers.