Perhaps Oyster readers at some stations are programmed to charge the
full whack, whilst others are more leniant?

If you only touch your Oyster Card at the start or end of a journey you
are always charged the minumum fare from that station at that time.
However, it will not count towards any price capping calculation.

So if you have been capped at Zone1-6 and make a further journey
where you only touch at one end, you will pay more than you need to.