Because, being the Frame fullness % of that last page, below the threshold/s for the Vertical Justification (in Layout Options/ Vertical Spacing), Dorico doesn’t justify the last page. A fast way to adjust this is to drag (in Engrave Mode) the Music Frame lower edge a little higher, so that the frame shrinks, and the fullness percentage will be so that the threshold will be reached and the justification applies.
For the whole layout you can deactivate Automatically resolve collisions between adjacent staves and systems (in Layout Options/Vertical Spacing/Minimal Gaps), so that only the settings of Ideal Gaps will be applied.
I suggest you this recent video about layout: