A level 10 keep requires 1 full stack of 200 Ironwood per repair. If I have a full bag of 8 stacks, each repair consumes one full stack...until I get down to 2 stacks remaining. At that point I get a message that I need 2410 more units of wood.
Somehow, keep repairs are either
a) removing less wood from the player's inventory than they should for each repair, or
b) requiring more wood to be in inventory than the repair will actually consume
This is well documented and easily reproduced. Simply find a level 10 keep that needs repairs. Have 3 stacks of Ironwood. Make a repair. Try a second repair. Basta!