Roland Perry wrote
on Mon, 16 Apr 2012, Michael R N Dolbear remarked:

Cite please.

Previous explanations of Oyster capping here and elsewhere showed
this
working as would be expected, with no cases where using two PAYG
Oyster
cards would give a lower cost.

So how can we explain the OP's problem, where it would have been
cheaper
to charge for a Z1-4 plus a single journey 4-5?

No idea, perhaps submit a FoI request for the number of refunds that
invoked the Oyster G'tee ?

If it was just a glitch, I may remark that in my thirty years as a
System programmer I proved the existence of a hardware error (that
didn't crash the system) precisely twice.

--
Mike D