The reason the feds would offer 12 months (rare) would be for 2 reasons.
1) they build a new case while he is inside.
2) he gets 12 months and another 12 months on paper with the hopes of catching him spitting on a side walk and he gets violated.
Its obviously a weak case. If the feds dont feel they will get a conviction, tbey should never waste tax payer dollars and go to trial. Its unethical.