Practice and increasing confidence (which comes with practice) may be the key here.
I agree with Val and think this is Normal.
My ds12 is in Honors Algebra I. He has struggled a bit with silly mistakes but is getting much better. It seemed half of his mistakes was an absent or misplaced negatives. Other mistake were very simple calculations also. He gets plenty/a lot of homework but struggles sometimes with doing only what is assigned and not reviewing on his own. Again, recently he is doing much better and out of the blue his hand writing is much better and organized which is probably helping.
