CR123 calculation is wrong. First, voltage of CR123 is rated at 3V. But under load of approximately 2A you get a voltage of about 2.25V: Test/Review of Panasonic Lithium Power CR123
So 2 of those in series you get 4.5V, and P = 4.5V x 2.2A = 9.9W.
This is the correct value for something like an XM-L2 emitter producing about 1000 lumens. This can be seen again in Cree’s specs: XLamp® XM LEDs Archives - Cree LED
It says 10W @ 3A
Of course you still need to take into account driver efficiency, but I’ve excluded that for the time being.
So your light is working fine using 2xCR123 cells, but is doing something wrong using a single 18650 cell.