I see the same problem as Ares

coffers are what contain the generals equipment, is this the coffer you are talking about or coalition chests which are something different 

Ares said:

In the new deed rewards feature in CC, you have added deeds (thank you).

One of the reward categories is 'Generals Coffers'

I understand this to mean chests?

Firstly why use coffers when in game you use the word chests? Simply confusing.

Secondly there appears to be a bug.

Completing the other categories, I redeemed the chests awarded - about 6 or 7 chests.

After opening these chests - 'coffers' reward count in some of my cities showed the 6 or 7 coffers opened and in one of my others showed 1??

i have seen this same miscount when training cavalry unit and when training scouting units.

The last Deed (i think) is Named "General Coffers" this means that you must Open (5 of them) General Coffers to complete the deed.