Enter 125 GB instead. The hidden Recovery HD is what's causing this.
Shouldn't the space be already allocated for it in the current Mac OS partition?
It's not though.
I don't think this is the problem, I've never been unable to resize to all of the empty space by dragging due to the recovery partition and I've added/removed/resized multiple times while playing with Bootcamp. Even with a single partition there's no empty space where I could drag it, the partition "box" is full.
I've had the same problem as OP just weeks ago. Dragging the partition all the way down to resize (which resulted in 125.65 GB) never worked. Manually entering 125.0 GB worked because the Recovery HD takes up 650 MB.
I've had the same problem as OP just weeks ago. Dragging the partition all the way down to resize (which resulted in 125.65 GB) never worked. Manually entering 125.0 GB worked because the Recovery HD takes up 650 MB.