The 2011 collective bargaining agreement slots every draft pick into a tight salary range and each team has a cap limit on how much they can spend on draft picks which, in the Packers case, is $5,123,244 for 2015. Even the term of the contract is set at 4 years for all draftees, with first rounders having a team option on the 5th. year.
There's just a little wiggle room for negotiation.
The unsigned players are probably still haggling over some finer points relating to guarantees, bonuses and the contract structure.
For example, the team would prefer unguaranteed salary over roster bonuses which are also unguaranteed. The player would prefer the opposite. Roster bonuses are due at the start of the league year (around March 10) which is also when free agency begins. Roster bonuses force teams to make decisions early if there's some question as to whether the player should be kept or released; that's to the player's advantage.
It's not like the pre-2011 seasons when a player might hold out indefinitely and miss off-season/preseason activities when the stakes were much higher with no union contract provisions setting limits on what they might earn.
Agents have to justify their pay; haggling over a few 100k put in one place or another in the contract is about all they can do at this juncture.
OTAs begin May 27. Expect these players to be signed by then.