You should think of the ‘Ideal gaps’ on the Vertical Spacing page of Layout Options as minimum gaps (one of these days I should really change the wording), since unless Dorico ends up with a frame that is more than 100% full vertically, the ideal gaps are treated as the minimum gap that Dorico will allow. When Dorico is calculating the pre-justification height of the system, it adds the actual height of the staff (normally four spaces, of course) plus the relevant gap from Layout Options, and treats that combined total as the minimum height. So you should always set the ‘Ideal gaps’ values to the smallest value that you would be willing to accept, i.e. the smallest distance between staves that you would find acceptable; these values should typically be in the range of 3-6 spaces (i.e. from 3/4 staff height to 1.5x staff height), unless you want a particularly loose vertical spacing for some reason.
Dorico uses the unjustified height (calculated using the staff heights plus the relevant gaps) as an estimate when determining how much music will fit into a frame. It uses various other heuristics to improve this estimate, such as the presence of tempo items, rehearsal marks, lyrics, etc. in order to guess how full a frame will be. It cannot change its mind about this estimate if it turns out to be too conservative later, which is why you will sometimes end up with systems being too cramped on the page, requiring you to add a manual frame break.
Once it knows which staves/systems will be in a given frame, it calculates their precise heights, taking into account protrusions above and below every staff, and only pushes them further apart than the ideal gaps if there are protrusions that cause this to be necessary. This gives the final unjustified height, and the leftover vertical space is then justified equally between all of the staves and systems, taking into account the thresholds defined on the Vertical Spacing page of Layout Options for whether only the gaps between systems should be justified, or, if the frame is particularly empty, whether it should also justify the gaps between staves as well.