MAVS NEWS:

[F Gump]

But that's where he is today. Last season was ~$13.4M and next season is ~$14.4M. Why agree to extend at the same salary level?

Because (imo) 
1  he'd much rather be here, per the whispers
2  so he locks in a deal NOW at numbers that make DAL happy to keep him rather than trade him (and it's easy to structure the deal where he can't be traded right after it)
... or, to put it another way, if he DOESN'T extend, he becomes instant trade fodder, right?
3  with the hard cap squeezing salaries to fit within limits, what he used to be paid has nothing to do with the next deal -- a player gets paid at what a team will be willing to pay, AND what they can pay  
4 remember, Mavs didn't determine his prior pay, they have been stuck with paying it (in a good way or a bad way), but both parties know they won't be mentally obligated to base anything on that
5 sometimes player pay goes down
6 I think MLE (14.1M) is his upper limit if he tests free agency ...can't see his demand being enough to entice a cap room team to prioritize him
7 a signed deal now, in the same range as what you'd expect to get later, is a much better option, since all it takes is a major injury to gut value at contract time