Unexplained extra bit in FMA data path

Per section F.1.4 of the textbook, it would be good to understand the root cause of this discrepancy:

After this section was written, TestFloat revealed an FMA bug (cvw issue #578 (closed)) related to subtraction of a small value. Z_m^shifted had to be widened to Nf + 4 integer bits and 3Nf + 2 fractional bits to obtain a correct guard bit. The extra integer and fractional bit propagate through the rest of the FMA and postprocessing. The root cause is not yet fully explained.