Seems to me that the difference here is the representation. At least, that's my opinion. Understand, both of these agents are very high profile within the NFL. Dak is represented by CAA, which is France. There is a history there. France represented Romo and if we are all being honest, the Cowboys took a bath on Romo, in terms of contract. The team bet on Tony lasting and winning. Neither one of those things happened. France and CAA have a reputation and it's well deserved. They have a strategy and it's basically to negotiate the highest paying contracts possible. Not only that but it's also a strategy to go back and double dip on organizations, putting them in positions where they have to pay record follow on deals and they do it in a very short term. This puts a lot of financial pressure on teams and it's really not good.
Leigh Steinberg, on the other hand (Mahome's Representation), works differently. He is no stranger to the Cowboys organization either. Steinberg represneted Troy Aikman so the Cowboys know him well. Steinberg works differently. Steinberg believes in long term, career security contracts. Remember, Steingberg worked out the richest contract in NFL history for Troy, a 10 year deal that would secure Aikman's future. It's a very different approach.
Mahomes gets a record deal done and Dak is playing on the tag. I believe a lot of this goes back to the representation.