Staff Spacer Mystery

When working on layout, and positioning systems on an individual page, I will often use a Staff Spacer Up to push the top system further down on the page. In some cases, it will push the system away from the perceived margin, and in others it will continue on page the margin, and even the edge of the page, and on to infinity (?). This seems completely random.
I use this only when I want to adjust the layout on one particular page, and not, say, all odd of even page, in which case I'd just adjust the margin.
Compare pg. 2 m. 21 and pg. 5 m. 49.