Earlier today I did some PAYG experiments. In this post, 'travelling' is
defined as time spent in the 'paid' side of station facilities as well
as the train/tube.

I travelled from Liverpool Street 'NR' to Romford, then 23 minutes later
I travelled back. I wanted to see if I would be charged for two short
journeys or one longer journey, in this case it was the former that
happened (in reality I hit a cap by then but the findings remain valid).

After leaving a bus at Bromley South, I travelled direct to Victoria
'TOCs', spent just over 10 minutes in the concourse then travelled to
Waterloo 'NR 1 - 11' via Clapham Junction. On this occasion, it was
defined as one journey.

Less than 5 minutes later I travelled to Vauxhall 'SWT' then (again
within 5 minutes) took the Victoria line to its station namesake. I
expected my latest originating station would still be Bromley South but
it was now the aforementioned Waterloo variant.

Make of the above what you will, but my main question is this:

If ~5 minutes was seemingly enough for the system to end the ex-Bromley
South journey at Waterloo, why was ~10 minutes not enough to end the
same journey earlier at Victoria?

[N.B. I've noticed the Waterloo ITSO-ready gateline has been converted
to a 'standard' Oyster-ready gateline.]