Ah, I was missing that bit of information, and it explains why I was seeing these two values. Thank you stevage, I'll update the post with this info and cite you.
Thank you - I will watch that with my coffee. I also realize this means I set the extent to 512 in the mvt file - that means I won't benefit from that subpixel precision you mentioned - I'll update the code and test it out
FYI you might enjoy this talk I gave a few years back: https://www.youtube.com/watch?v=Zuo80hZYA1k