Thanks to forum user Feathers we’ve spotted that another rounding amonaly has crept in for late adopting National Rail services in zones 2-6. These are Southeastern, Southern, Southwestern Railway, and Thameslink (outside the middle). If you commute at peak times on a two zone journey outside zone 1 (so 2-3, 3-4, 4-5 or 5-6) then a paper anytime return costs £6.70 whereas a peak single PAYG fare is £3.40.
This will have happened because the rounding applied to the peak single fare in March was quite high whereas the rounding applied to the anytime return was much less.