[Gimp-developer] Mystery: scaling "thin" areas makes them partially transparent

[Ofnuts <ofnuts laposte net>]

I have a layer filled with black 100% opaque.

If I select a 3px wide strip only shrink the long side, the final result 
is not 100% opaque: I get a one-pixel high black line at 100% opacity 
with a black line at 87% opacity on each side. And is the selected strip 
is only 1px high, the resulting line is 74% opaque. If thereis no alpha 
channel I get gray instead.

This seems interpolation related since using "None" doesn't produce this 
result.

Is there an explanation to this? And can it be avoided?