My 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...)

Definitely 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.