The half bishops were the result of split bishoprics of which there were
two in England. One was the bishopric of Bath and Wells. Both towns
shared the seat of the bishop. Since both towns were in the same county
Saxton would count them as only one. The other split bishopric was that
of Coventry and Lichfield. Because Coventry is in Warwickshire and
Lichfield is in Staffordshire, Saxton was diplomatic in counting 1/2 a
bishopric for each. I'm not sure of the date that Coventry lost its
cathedral but it was around this time.
I think this clears up the last of your questions about the composition.
John Dwyer
dwyerjp@spot.colorado.edu