I studied this extensively and determined that the bank bag--which presumably was the largest version of three available, 28" X 14"--was too full to cinch. Based upon the volume the money occupied it left only 4" of bag to cinch at the top with the shroud lines. In other words, he had to remove some of the money to lighten the load in the bank bag.
The FBI, and Tina, have assumed that DBC tried to fit this quite large ransom bag into the much smaller reserve. He wouldn't attempt this because its obvious simply by looking at the two side-by-side that it wouldn't fit--there will be an exhibit demonstrating this at CooperCon.
I believe what actually happened is he used the dummy reserve to store several packets to lighten the load in the ransom bag so he could properly tie it shut at the top.