Common idea of issue:
k < m < n.
m - k < l.
l - visible size of list (number of visible raws).
k - current position of focus.
m and n - are ranges of interval, where all elements are selected.
If you are in k, and press ctrl+pagedown then, it is expected, that elements from k to k+l will be selected.
But, now, if you press ctrl+pagedown then elements from k to n + (l - (m - k)).

That means, that if some elements are selected (from m to n), than they are not counted, when number of shift in selection is evaluated.

See attached movie.
There are 18 elements and 4 is the size of visible part of list (4 raws).
I use ctrl+pageup/pagedown and you can see, how focus jumps on 4 elements each time. That is right.
After that I select some elements. Move focus to the end and start to press ctrl+pageup. You can see, that decrement is more than 4. It is more right on the number of the encountered selected items in the list.