I got it!

The distance changes based on the frame of reference.

If from the perspective of the stationary object, the two objects have
just separated by distance d, then from the perspective of either of
the moving objects, the distance has only increased by a fraction of d.
Length constriction makes the distance seem to be much smaller. It's
like saying 10 miles in one frame of reference is only 5 miles in
another.

Does that sound right?