It is not clear to me that the footballoutsiders website and the sportsourceanalytics twitter feed are using the same metrics. Did you see that somewhere?
In any event, the footballoutsiders metric for non-garbage possessions is a little odd.
Look at how he calculates 'garbage possessions' – if anything the calculation is undercounting meaningful possessions. None of your possessions in the second half count if you have a bigger lead than the other team has remaining possessions.
In other words, say that you've played a close game but thanks to a pick 6 have a 13 point lead going into the 4th quarter. The other team drives the ball down into the red zone, but can't convert, and kicks a FG. They're down by 10 with 10 minutes to go. You get the ball back at 25, and go on a patented CPJ death march drive, trying to eat up clock with the steady accumulation of first downs. You drive the ball 60 yds (to their 15) and miss a FG. The other teams gets the ball back at their 25 with 2:30 to go, needing two scores. They move the ball down the field but can't score as time expires.
Your drive was definitely not meaningless – indeed using up all that clock was huge. But on footballoutsiders' methodology, that 7:30 drive that gained 60 yds and essentially put the game out of reach, is a "garbage" possession. This is because at the start of the drive you were up by 10, and the other team ended up with one remaining possession ((8 x 1) + 1 < 10).
They're trying to account for drives where you just "kill the clock" to run out the game – but if you have an offense like ours that "kills the clock" as a matter of course, meaningful offensive production by GT is going to be under-counted.