As I was creating a website design I came across the following irregularity.. Please try and confirm the following:

1. Create a new document. Medium sized, I used 1000x1400 px
2. Create a rectangle in the horizontal center of the page, 720 px wide, stretching from top to bottom.
3. Create a dynamic offset of this rectangle by pressing CTRL + ALT + J
4. Set the offset's colour to black
5. Press CTRL + F and blur the offset by 1.0

Now stretch either the top or bottom of the rectangle and observe how the shadow conforms to the changes. At 720x1400 px, the offset shadow is slightly left-aligned compared to the rectangle. At 720x1010 px the shadow is RIGHT-aligned instead. I expect my cloned offset to be an exact copy of the rectangle but it seems as it shifts around arbitrarily depending on the dimensions of its source.

I'm using Inkscape 0.47

SOLVED: Silly me, I was using medium quality settings for filters and blurs. Apparently this causes the blur to misalign or something: it acts as expected on the best setting.