Out of curiosity I did a minor amount of research to get an idea.
replyTurns out that you are right, some balloon gas is 80%. Specifically, the "Balloon Time" tanks you can buy at places like Target say "not less than 80%" helium.
On the other hand, I went to AirGas and a few other suppliers and they seemed to have 95%-97.0% helium gas as their definition for balloon grade.
Perhaps "balloon grade" here is not "party balloon grade". Weather balloons? Research balloons?
replyMy guess is that places like AirGas aren't really supplying many weather or research balloons. I suspect the easier answer is 'Balloon Time is low grade crap aimed at people who don't know any better and just want to pick up some balloon gas while grocery shopping.' It's like the difference between people who go to a gas station to refill propane tanks, and people who swap them at Home Depot. (though the smart fellers do swap at Home Depot occasionally, if they need a fresher tank...)
replyDefinitely worth knowing what you're getting, in any case, so you don't get ripped off, and so you can actually get that lawn chair contraption into the sky.