The DB trusts were overcollateralized, meaning that if any loan underperformed it would have been replaced by WAMU from a designated pool of replacement loans. When WMB was seized this process was interrupted (breaking the repurchase contract) causing the process to cease and the Trusts to incur losses. That is why DB initially sued JPM and later the FDIC to recoup these losses since it was their actions (FDIC-R only as per the court) that created the issue.
NOTE: A couple days after DB filed the $10B claim, I personally read multiple articles where the author(s) predicted that the case would be settled for exactly $3B. They were 100% spot on.
Seems plausible that DB may have overinflated their initial claim, using it as a negotiating strategy, so that $7B claim reduction may not be that significant.