I think that's probably close to the tyvek weight, 1.85 / 16 x 150 = 17.34 pounds, vs 18.7 pounds listed as full weight.
Which means it's got a pound and a half of packaging, seems roughly right to me, depends on how it's packaged, if it has a cardboard tube core, etc.
I read the data sheets, it's not the same as tyvek, but tyvek and this stuff used some different tests so the results can't be directly compared.
This stuff is stronger though than tyvek, those test results were easy to get.
And given it meets the same standards for house wrap, one has to assume it has similar properties re water resistance and breathability. Wish I could get a sample though, this price is to me what it should cost for something so cheap and almost generic, tyvek to me is a very low grade material, and should be priced that way..