Overthecap and spotrac don't have direct access to contacts and notorious for being wrong. Spotrac even says right on their site "Our numbers do not reflect the actual real-time cap numbers provided through the NFLPA, rather an estimation based on the team's current contracts.." which is why I was going by the official NFLPA site.
In any event this was just posted to list a few easy painless ways to add cap space, not argue over which random website might have the correct cap number when nothing official has been released. I'll edit the OP and let you guys argue over that amongst yourselves.
No one's here to argue, it's an open discussion. Yes, NFLPA has the official numbers, but they haven't put up new numbers for 2014 yet.
We all use the same numbers from the same 2 sites to discuss what moves could make us money (like you have in this post). Just don't use them as a source and talk about how wrong the sites can be at the same time.
All of their math is clear at the bottom of the page. Cap Ceiling + carry over - top 51 contracts - dead money = Available cap. It's all there.