If you take (3) 2and a half inch blocks and sew them together..you should end up with an exact 6 and 1/2 inches....you should be able to iron, then measure them....this will tell you if your 1/4 inch guide is off(needle position, moleskin, foot) whatever you use to get your 1/4 "...if it's off, then investigate that.
But.....when making the same blocks on the same machine, even if it is off, they should all come together well because even 1/4" plus a "tad" will translate out to everything, and it should come together fine.
In your situation, if it was me...I would just accept it and go row by row and get them lined up...it is just sashing, and a tad off will never be noticed by anyone but you!