Equinox makes two kinds of tarps: a heavier "coated tarp" that is urethane coated and much less expensive and an "ultralight tarp" made of silnylon. The description sounds like you ordered the heavier one. But if it actually weighs only 16 oz then they may have sent you the silnylon one.

Either their shipping people are messed up (in which case you really scored a deal) or your scale is messed up.

Can you tell by feel whether the tarp is silnylon or urethane coated?