Sometimes the SB is calculated as a BB though so the first person to raise pre-flop in a 1/2 game could make it 8 instead of 7. Tends to keep numbers rounder and the math easier on later streets, especially if you are playing .25/.50 blinds.
still seems strange, because SB doesn't even have the full amount of the BB in the pot. so if they fold preflop without calling even the BB, your running pot count in your head will be off by 1 unit, won't it? and you're letting them bet more than what's actually in the pot.
The SB would always be treated as at BB on all streets so the running count would always round that last unit up. For example, 1/2 blinds and UTG makes it 8, Button calls, SB folds, BB folds. Now there is UTG's 8 + Button's 8 + SB's "2" (rounded) + BB's 2 (actual) = "20" in the pot even though there are 19 actual chips. Pot sized bet on the Flop would be 20, not 19, etc. Yes, the pot is technically off by a chip and the accumulation of errors makes the difference even more as more pot sized bets are made. Just makes it easier on everyone and speeds up the game accordingly. After I read about that option we changed over so .25/.50 blinds ended up being a $2 initial pot sized raise instead of the more cumbersome $1.75.