Freia's right. The max SP bonus is set at 10 levels. You'll get the same SP for killing a dodo that's 10 levels above you or 30 levels above you.
Janglo, you might have gotten a different amount of SP, but there are other factors at play.
1) Different mobs have a different "base" SP. For example, raptors and ahrimans have a "base" SP value of 150 SP per mob. On the other hand, coblyns and puks have a "base" SP of 63 per mob. So killing a colbyn that's 10 levels above you gives less SP than killing a raptor that's 10 levels above you.
2) Your party size impacts the amount of SP you get. Like Freia said, if you solo or duo, you'll get 100% of the SP per mob. If you trio, as long as everyone in your party is within +5/-10 levels of you, you'll get 90% of the SP per mob. With 4 people in your party, as long as everyone is withing +5/-10, you'll get 85%. Etc. So you could have received a different amount of SP based on your party size, or the levels of people within your party.
3) At lower levels, you get a bonus to the amount of SP you get per mob. Up through level 23, they take the SP you get from the mob, and they multiply it by some number. Between levels 1-6, they multiply it by 4.0. That number drops until you get to level 24, when you get the base SP (in other words, they multiply the SP by 1.0). So you could have received a different amount of SP based on your level.
If you want to read about how SP works, check out
Kanican's livejournal.
10.
It's +5/-10. If party members are outside that, your SP gets cut basically in half. But it depends on how many people are outside +5/-10, and how many are inside. And everyone's SP is calculated individually. So different people are different levels in the same party will get different SP.