I've noticed the same thing. A genset charge don't last near as good as a long "soak" with the turbines and solar panels. I don't know how yours works, but mine bulk charges and absorbs, then the inverter shuts the generator off. I have the absorb "done point" set at 2% of ah capacity of the bank, which is what the manual recommended. But I don't think the batteries are "quite there" yet at that setting and they require several hours at float to really get fully charged up.
Years since I set mine, so I'm working from a combination of memory of the original setup, plus the odd tweek since.
The inverter will call for the generator to run in a multitude of events. The only relevant at the moment are:
(a) if the voltage falls below (x)
or (b) if the amphours discharge is more than (x)
or (c) it's within (y) hours of the generator going into "night lock" and the batteries are discharged more than (z) AH
or (d) the generator is in "night lock" but the volts dropped below (z)
or (e) the generator is in "night lock" but the amphours are more than (y)
Once it's started, it will run for a minimum of 1 hour. If there's no other reason for it to turn off, and assuming there isn't more renewable comming in than some set limit, and assuming the total load is less than some other set limit (sounding like a nightmare to document already!), then it will continue to run until the voltage reaches the "absorb" setpoint, at which stage it goes into the absorb charge stage, which it will do until it gets to the "float" threshold. Then it goes into the "float" charge mode and will hold there for a set time before it turns the generator off.
It's very inefficient to run the generator just to hold the cells in float for 2 or 3 hours, so it usually drops the generator fairly quickly - I think I have it at 30 mins of float.
Even so, I think that 100 amphours of generator charge "holds" the batteries for less time than 100 amphours of solar charge does, assuming neither of them actually gets the batteries to float.