Non-pilot speaking
Maybe something is inhibited in the TL pedestal? Or software?
It would be helpful to get this answer. I asked this question about 500+ posts ago. What are the different ways in which Boeing and Airbus 'lock out' a reverser?
If the locking out is done by a mechanical latch/bolt on the sliding reverser cover AND in software, it would mean that the TLs could be handled in an identical way every time. However, since that sounds too simple, I am sure that it has been thought of and discounted. Perhaps leaving the one TL at Fwd Idl is a left over from the way it was always done.
I realise, of course, the enormous implications of making such a change in system and operational procedure.