I have given up on figuring out why some rides are harder than others. For example, I did a 58 mile ride recently with slower riders, it was pretty flat, with a medium headwind on the way back. I was really tired at the end.
But a few days later I did a 61 mile hilly organized ride. I rode really hard -- essentially racing someone else. I felt great at the end, and felt I could have ridden a lot farther.
I've often noticed discrepancies like this.