In article ,
Matthew wrote:
Or is it the scenario where I have £5.10 left and make a £7.30
journey to Watford Junction from Zone 1 that leaves me in negative
balance that makes me unable to enter the system again.

I suspect it's probably because they don't really like cards having
negative balances if they can possibly avoid it. So they'd like to
ensure that you always have enough money on the card to make that 7.30
journey *without* going overdrawn.

-roy