Thinking about it further, I think there are another couple of factors in my high success rate:
Firstly, a lot of my punting is with Thais. In my experience, Thais tend to be far more customer focused than most other nationalities (certainly compared to Roms and Brazilians), and I have had a much higher success rate with Thais than other nationalities. While there has been some slight creep in Thai prices, they still offer excellent VFM (the agency rate is still £150 ph in London, compared to typically £300 ph now for EEs, for example).
Secondly, over time I have come to realise that service is far more important to me than looks. I can overlook that a girl isn't a stunner if she offers a stunning service.
As I said, in my current circumstances I have to make each punt count. I think a 60% success rate would not be worth the effort I have to make to get somewhere I can punt. When I make a punting trip to London, for example, I can do so in the near certainty that it is going to be a success. It is only on the odd occasion when I go off-piste and try someone new do I sometimes come unstuck. But even then, that will be made up for by the other successes I am certain to have on that trip.