Yes, it's been a thing. Many airlines have ticket categories - maybe 20 tickets at this lowest price, and then another 20 at the next price level (strictly my example!). You are probably crossing a threshold. If you only buy 2, you're in the lower price category. If you bump into the next category, they'll charge you the higher price for all 6. You could probably buy 2 or 3 at the lower price and then buy the rest at the higher price. Then again, just when I think I have airline pricing figured out, I really don't! But we have definitely seen this situation.