I've 2 cats.
Evil Cat: main hobbies are planning world domination, stealing half plucked wood pigeons from the local Sparrowhawk, chasing innocent (and bemused) dogs and pretending to be friendly before attempting to gauge my hand and bite me to death. Also has a penchant for 'knocking' on the wardrobe door to wake me up when she fancies a 5am snack.
Then there's Ginger Boy. He's ever so stupid but incredibly handsome and very friendly.
Take them for their yearly injections today and I pre-warn the incredibly handsome young vet that Evil Cat is, in fact, not to be trusted.
She then proceeds to flaunt her furry backside around, behave impeccably and even EVEN let me stroke her and hold her while she had her injection.
WTF is that all about???
She has form. She's long haired and I take her to be groomed once a year (I can manage it mostly but in winter it gets matted), again I pre-warn them. The first time I took her, I did the spiel "she can be friendly but she is very vicious too" I go to collect her, worried about what I'm walking into (blood spurts up the wall, Dexter doing the blood spatter analysis) and am told she's one of the friendliest cats they've ever met, how lovely she is and she's a joy to groom 
Why is she such a hellbitch with me??*
I'm so lovely to her.
*unless I'm ill or pregnant - then she loves me and is my shadow.