For some reason the vertical distance between the two last systems is slightly smaller than the distance between the ones above. I want to move the last system down a bit. I guess Engrave mode, but then what?
Likely because of material above/below these systems. If you post the file here, someone can figure out what.
Staff Spacing mode. (But Dorico likely has a reason for spacing things that way.)
Thanks, Aaron!
I also just noticed that the space between systems on the penultimate page is bigger so I will try to match that distance to make it look better.
I won’t bother anyone with looking through the file and the topic is solved, but since we have discussed it, this is the spacing of the penultimate page. Lots of space here.
The last page has these settings although there are both higher and lower notes that make everything narrower. If anything it should be the other way around regardless of preset settings and so on. From my point of view.
I’m going to change the last page’s distances to 25.6mm.
Only by seeing the file can people explain to you what is happening (and why).
Dorico does not make these spacing decisions for no reason (and posting some tiny image of the left margin does not help at all)
Those are results more than settings! 
The settings in Library determine how the program will calculate the spacing which show up as the dimensions we see in the left margins.
We can only guess without seeing the file or at least more of the page. If you would like to know why this is happening so as to not have the same problem in the future (or have to use a solution that may be less than ideal) we would be happy to help; but we need more than that limited picture. (…as @Janus suggested!)
– Jim
Just in case you haven’t read this section of the manual: https://www.steinberg.help/r/dorico-pro/6.1/en/dorico/topics/page_formatting/page_formatting_staff_spacing_c.html
It explains clearly how the vertical spacing works.
Without seeing the file, I can only guess that either there are more systems on the last page than previous; or vertical justification isn’t kicking in.
Thank you, Janus, Jim, Rikard, benwiggy!
Maybe I’ll make an example file eventually.