Am I getting the true sense of the word, poser, here, or am I missin' something?

Think a combination of:
one who poses, ie gives the impression of being something they're not, and
one who imposes, ie tries to fit in somewhere they don't belong.

As mentioned above, it often implies impurity of motive. Think Robbie Williams as the "bad boy of rock" - he acts bad on occasion because he is the "bad boy of rock", and not vice versa.