Your choice of decking was irrevalent because you stuck to the standards for joist spacing- if you wanted to space your joists 24" o.c., your choice of Trex decking would not be approved unless it was 2" thick product, or if you ran your decking at a 45 degree angle to the joists- you'd need to go 12"o.c. for the 5/4" material.
Thinner decking needs closer spaced joists, and closer spaced joists, if properly sized and supported by adequately spaced and sized beams, which in turn are supported by adequately spaced footings, all equate to what load a deck will bear.