I think the 5-14 stat is just what the unidentified ring states. I don’t know why it’s not the same as the identified +0 stats.
ETA: I took a closer look, it seems like the scaling is fractional. At 10 STR, max dmg goes up by 2.5 per upgrade (rounded), then at 11 STR, it 3.5.
Bow gets +1 per 5 Lv., ending at +6 at Lv. 30. CI grants upgrades in proportion to original upgrade. +0 to +5 gets additional +1, and +6 should get +2. Try using CI again.