Let's say a 7th level halfling rogue with Int 12 is our subject.
He has nine skills, each with 10 ranks in it. That's ninety skill points
Then he gets hit on the head, and his Int goes to 12. His total skill points drops to 80. He's behind by ten.
IMC, when he goes up a level (to 8th) his total skill points goes to 88. He can't buy any skills, because he's still in the hole.
When he goes up to tenth, his total goes up to 96... now he can spend six points.