This behavior is caused by the dataset. When grid.PageMode = true and DataSetType = dtNonSequenced the grid only displays a buffer of visible records, the grid has no information at all where this buffer is positioned in the database except that it is the first buffer, last buffer or not the first or last buffer. Hence, there are only 3 possible scrollbar positions.

When you set grid.PageMode = false, the grid loads & shows all records and can thus show an exact scrollbar position.