A common fault for a battery that sits a lot, or even those that are driven, is that the cells can die in it. That will means it will still charge up fully, tell you it has the full 12V and stop charging, but only to the capacity of what remains. I can't recall if that has the same cranking symptoms in an older car like a flatter battery does, been a while. But one thing to think of next time it gives trouble, if you have another battery nearby to test same with.
My father had this issues only 2 days ago with his older 70's Volvo runabout. He still drove it almost daily, just plodding about town and at no great speed. But it would get slower to start, he'd put it on charger which said fully charged in no time, and the issue continued. It ended up having a dead cell or two, full 12V but not enough cranking amp capacity to do anything, and the chargers switch off when they reach full. Some have different options, but you can't fix a dead cell no matter how long you have it on a charger of that type. Perhaps if you started doing a long of driving for long periods and at speed/higher rev rate to keep zapping it at 14V or so. But long term it will give issues again and again.
