How often? Every time... and the gap gets wider at 4th level. And again at 8th level. And again at 12th level. And again at 16th level. And again at 20th level...
Every time what? Those ability mods are equal. The gap begins at 4th, but see below.
Really? So if you had all odd stats you would raise each of those abilities at your ability increase. Somehow, I disbelieve that is the consensus.
Wizard -1st level 17, 15, 15, 11, 11, 9 [36 pt buy]
Str 11
Con 15
Dex 15
Int 17
Wis 11
Cha 9
Wizard -1st level 16, 14, 14, 10, 10, 8 [26 pt buy]
Str 10
Con 14
Dex 14
Int 16
Wis 10
Cha 8
So at 4th level the 36 pt buy has an 18 Int vice a 17 from the 26 pt buy, all other stats barring esoteric feat choice are equal. At 8th level the 26 pt buy wizard catches up to the 18 Int, and you're saying the other wizard chooses...what? Hp? AC/Init? Fine, the 36 pt buy character has a little more hp or a little more AC/Int (or whatever you chose to put that point in). 12th the 26 pt buy puts it in Int, 19. Your character does what? Matches pace?
Its a simplistic example but it shows my point. If you choose to round out your ability scores, you will generally have a weaker specialist role than someone else, even if they are 10 pts less than you on the point buy scale. Since you now have differing power levels, it becomes even more muddled whether one is 'demonstrably weaker' than the other.
Point-Buy is not perfect and not the end-all be-all when it comes to Balance.