In message , at
21:05:16 on Mon, 14 Jul 2014, Tim Woodall
remarked:
Z1-Euston Square on a second PAYG card 2.20
Euston to Watford Junction on my main PAYG card 3:10

Doing it all on my main PAYG card cost me 8.90
....
As an aside, I thought using oyster was supposed to guarantee the
cheapest journey. Even without the gold card, doing my journey on two
cards is 6.90.

All they say is that using Oyster is cheaper than either:

*a* paper ticket (probably some form of Travelcard) that would have
enabled all your journeys that day.

a series of paper singles.
--
Roland Perry