Seems just plain sloppy to me. Its not hard to measure these things.
For a stub in the middle of the slab, measuring over a 20 or 30 foot
distance, combined with the jossling of the cement pour, being off by
an inch or two seems pretty reasonable. For stubs near the edge
(probably the bathroom toilet and kitchen sink, for example), an inch
or less seems easy. But 3 feet? Most plumbers should be able to eyeball
to within 3 feet without even measuring. Anything more is sloppy, or
(on the occational pipe) just a simple and forgiveable mistake.

-Kevin