As is so often the case with NFL contracts, things aren't as simple as they appear to be. The deal Jackson signed was very team-friendly.
ESPN's Dan Graziano notes that the Eagles can get out of Jackson's deal very easily after two years. Jackson will earn over $17 million over the next two seasons, $14.75 million of which is guaranteed, including a $10 million signing bonus.
That helps explain why Jackson signed. But keep in mind, Jackson needs to produce at a very high level or the Eagles can easily get out of the contract after two years. Jackson's salary jumps to $10.25 million in 2014, but only $250,000 of that is fully guaranteed.
The Eagles were already slated to pay Jackson $9.5 million on his franchise tag this season. With that in mind, this is a terrific deal for Philadelphia. The Eagles get him over the next two years at a cheaper average rate, even if it provides more security for Jackson.