I agree about the intent, if for no other reason that it would make absolutely NO sense to make a first level spell that's not of any use until fifth level.
When I first read it I assumed it was a minimum of +1, so it would be Levels 1-9 = +1, Levels 10-14 = +2, Level 15+ = +3Levels 0-4 = +0, Levels 5-9 = +1, Levels 10-14 = +2, Level 15+ = +3
I vote YEA for Levels 1-5 = +1, Levels 6-10 = +2, Level 11+ = +3