The ornamental borders are intended to work with any page size, in theory, but as you've discovered, theory and reality don't always match... The corner pieces have a fixed size (set from the borderwidth), and it then allocates what space is left as best it can, according to the rules it's been given.
For Han4, for example, the sides are made of an up-facing and a down-facing element, (depending on how you consider the 'face' of the little repeated units!) and the rules say to put the change of direction in the exact middle of the spread, and then increase/decrease the number of elements above and below the line as makes the most sense, with stretching and shrinking them being permitted as well. But it's not allowed to leave any blank space, or shrink the corner pieces. I guess that you have some pages which are ending up so short (and the border width is so big?) that there's really only space for half an edge element and it's trying to put one up-facing and one down-facing in there.