Basically in the past, the team had a rookie pool and was free to spend as much of that pool anywhere it pleased. Now each draft slot has a set percentage of the rookie pool associated with it.
Teams are still free to spend their rookie pool however they want. There's nothing that says any rookie contract has to be for any specific dollar amount.
The reason why most of the contracts are basically slotted now is because the NFLPA "accidentally" sent the agents the rookie pool values for every pick last year, so the NFL sent them to all of the teams. The CBA says those values were never supposed to be revealed. Now that everyone knows what they are, however, both the teams and the agents have pretty much settled on those values. Last year, there were more contracts that didn't use the set values, but this year, they've almost all been just about exactly by the numbers.