I'd probably bet that most who say its a mk5.5 are mk5 owners and people that know nothing about the mk6 and are just feeding off info from the the other bandwagon jumpers.
For those of you that don't know, and for those that are too lazy to check for themselves, here are some differences.
*note, not all changes are necessarily better, just noting the differences*
The body is different from the panels, to the lights, to the grills, the exhausts etc, they are different. Alloys (shadow). XDS differential, updated ESP, DCC/ACC, a different engine (ok, not totally different but modified TSI over FSI), its lower to the ground than the mk5, bi-xenons, different interior with better quality parts used, its cleaner, greener and more fuel efficient, and cheaper on your pocket due to less tax banding, engine is quieter, sound generator, sound dampening when driving is different and thats all i can think of from the top of my head at the moment.
Of course one or two touches are subtle and I totally understand that for the glass half empty mk5 owners you are trying to justify in your head that your car is still really good and the best, well sorry its not, nothing lasts forever. Dont get me wrong, the mk5 is still a gem of a car, its just a shame their owners were not still as classy.