Quoting: Bruins1054_gmb
They're both eligible for 35+ contracts, which means they could sign for a smaller cap hit with incentive money in performance bonuses that wouldn't count against the cap until next season. So, it isn't ridiculous.
What is a Performance Bonus?
A Performance Bonus can be given to a player in addition to their salary. Performance bonuses are only paid to the player if they meet the requirements as defined by the bonus. Performance bonuses count against the salary cap; however, a team can exceed the salary cap due to performance bonuses by the maximum performance bonus cushion amount of 7.5 percent of the upper limit. Not all players are eligible for Performance Bonuses, and to receive a bonus within their contract, a player must meet one of the following criteria:
1.The player is on an entry-level contract.
2.The player has signed a one-year contract and is over 35 years old.
3.The player has signed a one-year contract after returning from a long-term injury (has played 400 or more games, and spent 100 or more days on the Injured Reserve in the last year of their most recent contract).
Bergeron signing for 2 years wouldn't be eligible as per point #2.
Contract has to be 1 year for it to be eligible.