Hello,
It shows in the cargo capacity tool-tip: The -900 has a total cargo hold of 170m3 vs the 77E's 160.30m3...
That would be the reason. Basically, when you fly aircraft with high payload capacities and limited cargo room, you stick to a cargo ratio that favors taking STD cargo. So for the 77E, something like a 20/80 or 30/70 ratio would be a good choice to maximize your limited cargo hold.
Talentz