It seems it isn't currently documented...
https://wiki.lazarus.freepascal.org/IDE_Window:_Watch_list#Viewing/Expanding_ArraysThe embedded toolbar by default works inside the range of the returned watch (in your case 0..20).
That is, so that the last page ends at the end of the array. If your 21 entries were paginated on pages with 10 elements, then the last page would only show 1 element. Sparing you the random data after your array. (for the toolbar, this is not distinguishable from a real array, and even if it was it would not know if you had chosen the maximum value for the index)
However, if the start point is forcefully set outside the bounds, then it will give up enforcing those bounds.
So in your case, if you set the start or -1 or 21 then the paging will show more values.
The arrows will not set those values. You have to type them directly into the edit fields.
Since the default for pagination size can be bigger than the array (e.g. an array with 5 elements, but pagination starts at 10 per page), the page size is not used to determine if bounds should be ignored.