The replacements are not truly "refurbished".
Apple calls them "remanufactured" units.
The difference?
In most cases, a refurb means the unit has been brought back to a facility and cleaned and tested. If it tests good, it can be resold or used again.
A remanufactured unit usually means it was taken apart and each part individually tested. If the test was good, that part can be used again to build an entirely different unit with other reman'd parts. These are normally higher quality replacements compared to a refurb.
Each time I've been to an Apple store, I've always heard them state that the replacements are reman's and not refurbs.