Is this on a breadboard?
yes & no --actually I not sure for those particular images. I did testing with my soldered board & a breadboard & saw no change.

If you are using it where I think you are - generating the 1.65v offset voltage
my understanding is the internal reference is 2.5v (2.49...) and the way I stumbled into making 2.6v just kind of seems wrong to me. I am not gonna worry about it to much, unless the neg regulator I choose to use shows the same irregularity.

I have been sorted my electronic comps and found a few adjustable negative voltage regulators---that's just going to create problems right? better to stick with something fixed?

Well, it's a cleaner design when there's no extraneous errors... even if they don't end up affecting the final end product in a detectable way (i.e, the machined parts come out on-size with good surface finish). What I'm saying, I guess, is that if you can't squeeze out the last little bit of error... don't freak out about it.
noted...but I might need a reminder on that one, my OCD keeps me trying for perfection, even when its not at all worth it.