It probably has to do with your settings in Layout Options > Vertical Spacing > Vertical Justification.
If you switch to Engrave mode, you’ll see a number in a colored box at the lower left corner of each page. I’m guessing that the second page is between 60 and 80% full, but the other three are over 80% full. Based on the default settings, Dorico adds space between staves and between systems on p.2, but only between systems on the other pages.
If you change the second value (“Justify distance only between systems”) to 100%, then Dorico will always apply spacing between staves and between systems, as long as the frame fullness is over 60%.
In Veritcal Justification I set both to 100% and turned off the ‘Justify Staves when frame with single system’ check box. I now have this. Is this as close I can get to even?
I would leave the first value at 60%. That will get Dorico to apply the same kind of justification to all pages. By setting them both to 100%, you’re telling Dorico never to apply justification.
Both settings only activate justification when the frame is completely full; in other words, it will never happen (or will make no difference to an already full frame).
Result: Our 60%-full frame is not justified, and the values in the Minimum Gaps settings are used.
Both settings activate justification when the frame is empty; in other words, the frame will always be justified. However, the second option takes priority.
Result: The systems are justified by adding space between the systems. The staves are spaced using the Minimum Gaps settings.
The 60%-full frame falls above the threshold for justifying staves and systems (50%), and below the threshold for only justifying systems. So staves and systems are justified.
Result: The music is justified by proportionally increasing space between the staves and systems.