But they weren't always credited for them. As for why they would be credited with those Skill Points, because they earned them.
What is it that you don't get about that logic? If I'm a 38 Goldsmith and people are getting credited extra SP earned between 11 and 31 (based on new points), why on earth wouldn't I expect to get credited with them? I've earned thousands more skill points to get to 31 than is now required. Given that jobs between 11 and 31 are getting credited with that extra SP, why wouldn't jobs above 31 also get credited?
I'm astounded that you cannot follow that logic.
No one is being credited with any SP. You are only confusing (and frustrating) yourself by looking at it that way. Ranks are not based on the total amount of SP you have accumulated. The only thing the game tracks is the amount of SP required to obtain your next rank. The amount required, for ranks 11-31, has been reduced. This will make progressing through those ranks easier, for those who have not already done so, to the extent they have not already done so (ie. a rank 20 will get less benefit from the change than a rank 10 would, and a rank 31 will, obviously, get no benefit from it at all). In fairly rare cases, it may even result in a player getting a new rank immediately the next time they gain SP, instead of having to wait as much as several thousand more SP for it, depending on how close they were to ranking up based on the old SP requirements for that next rank.
The purpose of this adjustment is to make it easier to get from rank 10 to rank 31. You are already rank 38, so the adjustment does not apply to you. You did it when it was hard(er). Feel free to brag about that if you like, but the bottom line of the story is that this adjustment is not intended to have any effect on jobs that are already ranked above 31, and it won't.