Peter,

I suspect the OP actually constrained his clocks in ns, rounding
266Mhz to 3.75ns. That would explain why the STA calculates a 250ps
distance (which is correct for 3.75ns and 4ns clocks).
If not, this would show a bug in STA since the distance (i.e.
"Requirement") between the clocks should then be 0 (after rounding).

The issue though lies in declaring these clocks as unrelated,
typically with false paths.

- gael