That is primarily because the 2011 NFL Collective Bargaining Agreement, negotiated by the NFL Players Association with the NFL Management Council, agreed to a stringent, inflexible rookie salary cap. The team can extend that contract after the third year by paying a new signing bonus and adding veteran salaried years after the first four. Another negotiable issue is the timing of signing bonus payments. Teams attempt to defer portions of a large signing bonus. This is why the signing bonus was at such a premium.

Source: Forbes June 06, 2018 19:07 UTC