Hi Martin
I don’t think it’s related to the March fare changes, as it seems to have been the case for well over a year (and followed the same pattern before the fare rises last year – i.e. 20p “premium” for EL).
e.g. taking a random day from my history with the same journey profile over 6 months ago:
I’ve asked TfL on a couple of occasions why it’s happening, but after the initial response to say they’re looking into it, I get nothing back.