I was farming killing moles and I stumbled across a discovery I thought I would share.
I am not stating this as fact...
It is generally thought (from what i've read and heard) that SP is gained by the amount of damage you do to the mob and also a randomizing factor. I think this is only part true.
It seems like, from my farming experience on moles, that its not the damage you do per-say, but the amount of health you take away from the mob.
Now you might say, whats the difference! well theres a big difference. Using a "big finishing move" will actually lessen your SP gain over time. Also, the reason I think it's set up this way is so that no matter how uber strong you are, the monster only has X amount of health and only X amount of SP to yield. of course the randomizing factor still exists as well.
How i came to this conclusion was as follows. If you've ever been in combat with moles then you know they tunnel underground. Well, if you do a weapon skill right before it tunnels, you cant kill it because its under ground and cant do the dieing animation so when it pops back up it only has 1 health left. EVERY TIME this happened, I got 1 sp (when i got and sp at all of course).
Therefore, the logical conclusion is that SP is linked to the health of the critter and not the damage you do. After i noticed this it was pretty easy to recreate the situation where it had 1 health left, and at that point i killed using regular attacks and weapon skills. every time i got SP, i only got 1.