UPLab is not a “slight tweak” of CIELAB. It’s a lookup table for going back and forth between CIELAB and a slightly rotated version of Munsell space, https://en.wikipedia.org/wiki/Munsell_color_system
Explain your "slightly rotated" comment?
Also, to be completely precise, it should be noted that the lightness dimension of UPLab was left the same as CIELAB’s lightness, rather than using Munsell value. Only a and b were adjusted to reflect Munsell hue/chroma.