Hmmm...
I mean the obvious answer is whoever was doing the props just molded off a couple more of those, and they fit the more typical human build, but I can't honestly think of any sources saying anything about why humans had on similar armor...
It'd be easy to say though, that like the Gammoreans who are all dressed the same, that it's something of a uniform for the human guards... Maybe issued by Jabba's own armory even, but it could be the humans are their own group too... Helmets have sime similarities too (I think the guy who shoots Luke's hand, and Lando's bucket, are similar at least... Frozec may be sporting parts of the same headgear.
The whole Rodian thing always weirded me out... They love that green racing jumpsuit with the stripe, and a tan vest of some sort. They're like Nacy's closet from Nancy & Sluggo. You just expect the same outfit times 12 hanging up in any Rodian's wardrobe.