I had 3/8" thick Bellawood installed today over concrete slab (with a skim coat of a prep material). There are many small gaps between boards side-by-side, plus a few length-wise. Is this simply poor workmanship or is the installer correct when he says that these gaps are needed for future expansion?

I am a bit dubious of his claim because we are in the middle of a hot and humid summer in the Gulf South. It seems to me that the boards should be as expanded as they'll ever get right now. (Unless the levies are breached and New Orleans floods again.)