Emergency O2 tanks shouldn't be pure O2

[Marquesas]

I'm confused. I haven't had an emergency O2 tank last an entire round since donut. I always set mine to 21 intake (any lower and I start gasping) and it lasts me about 20 minutes. I always fill mine to the brim at an P2p tank, too. am I mistaken about something here?

Stop breathing so heavily, an etank at 21 lasts for pretty much a whole round for me.