The 20 percent lighter thing matches what I have been seeing too. I keep a few scraps from old jobs in the truck and the difference is wild when you hold them side by side, same label and everything. My knees are shot by 2pm on the new stuff and I have been telling customers the pad is where I would spend the upgrade money now, not the carpet itself. You are not old and cranky, you are just paying attention.

Last July in Phoenix I started dipping my trowel in a bucket of ice water between blocks instead of dumping water in the mix. Keeps the mortar from cooking on the board without messing with the ratio. Also started mixing half batches, like you said, and setting the wheelbarrow in the shade with a wet towel over it. That got me through the whole west wall without losing a single block.