Isolating VTG on the STK500

I wonder haw many have been bitten by this one?
With the VTARGET jumper in place, VTG is 4.9v. Nifty!
With the VTARGET jumper removed, VTG is 4.1v, and the target still runs!

Removing all the on-board ribbon cables made no difference. Pulling the
AREF jumper finally powered down the target. Eureka!

Sure, there's some waffle in the STK500 user guide section 3.8.1, about
relativity of VTG and AREF, but no admission that AREF will power the
target in the absence of VTG.

Mutter, mumble, but no smoke, so shouldn't grumble.

Erik