Scaling with snapping

When i select scaling with
snapping set to 1 and
select ‘preserve non-uniform scale’
the following happens.

take a cube of size xyz(1,1,1)
set scale value of y to 1.5 in editor (not gizmo)

when i scale down y (with te gizmo) y is set to 1
when i scale up y (without letting go mouse button) y is set to 2.5

Where do these values (1 and 2.5) come from?