I can't see a great level of detail from your photo, but it is usual for new brushes to arc a fair bit, until they're run-in.
There is a processes called dressing(?) which speeds-up that settling-in period by grinding a hollow in the brushes to match the curvature of your particular commutator.
The worn groove in you commutator may have been caused by the brush-casings when the brushes wore down to slivers, depending on their design.
I would expect your new brushes to wear quickly where they're arcing now, until they match both the commutator curve and groove, then they should settle down.
You can gently sand your commutator with very-fine abrasive paper. That will remove dirt, glazing build-up, and smooth out the groove. So long as there's enough 'meat' on it so as not to go through the thin copper entirely.