Hmm... I'm not quite sure about "the opposite" here - Dominion vessels were not outright said to be better shielded than other opponents of Starfleet, if that is what you mean.
True, the Odyssey
did not blow up even the smallest Dominion vessels with one shot, or three, or five (quite possibly, one attacker was
destroyed eventually by the starship, but two assuredly were not). But at that point, rules of engagement may have called for wounding shots, similar to the ones that so often were fired by Picard. Thereafter, we saw a fairly ordinary succession of individually seemingly impotent hits accumulating into a simplistic fireball destruction whenever the Alpha fleets fought their Gamma counterparts.
In contrast, the dialogue evidence for the vulnerability of Alpha shields is clear, and the elimination or at least alleviation of the vulnerability is something of a plot point in "Call to Arms".