While having Ghengis Khan with some dou-min, one of them asked for "biiru hitotsu". While I could understand the biiru, I was surprised that ippai wasn't used.
I understand that (number)-tsu is valid Japanese, but not why it was used in this particular case. Is using a more vague counter word more casual and relaxed, akin to saying "a beer, please" rather than "one glass of beer, please"? Or would "ippai" be confusing because of the phrase "o-naka wa ippai (desu)" being used to indicate you're full?