Nothing funny about it at all. Many of the 38% of US households presumably own more than one firearm.
There is nothing to indicate that a non-gun household would be any larger, or smaller than a household with guns. Thus, if all households (gun and non-gun holder) are about the same size, and each household has 4 guns; this does not work out anywhere near 90% of the population having a gun.
The numbers are wildly different than what the author is alledging. I submit that the true number of guns per American is somewhere between 38 and 90%; probably close to the 60-75% range.