MRC tuning (Audi RS gurus) believe that some shell stations (which are all franchised) may have sold 95RON as 99RON. Would take some collaboration with tanker drivers. This is just a theory based on their observations.
They have told me they've had instances where they've had cars making significantly lower power on their dyno on shell 99, dumped the tank and refilled with Tesco 99, and hey presto power is back. They had one such instance with a tuned RS6, and the power difference was almost 50hp.
I always try to go for Tesco 99 when I can in my Audi. However, I've found the Mk5 GTi seems quite happy on 95. It dyno'd at 197hp recently on 95. The engine was standard at the time, has a fair amount of carbon build up, and was dyno'd with bonnet shut. Quite happy with that. It has now been bluefin'd, so more inclined to use 99RON.