There seems to be the following off-peak single fares to/through Zone 1 from Reading. This list is not complete, and it there is no doubt further weirdness!
Reading – Paddington: £12.60
Reading – TfL-LU scale stations in zones 2-6 via Zone 1: £15.40
Reading – Elizabeth line (not Stratford) and NR-scale stations in zones 2-6 via Zone 1: £15.60
Reading – Zone 1 stations: £15.70
Peak is a bit more complicated, but seems to be in the right order that going to zone 1 is cheaper than going through zone 1 – unlike off-peak!