I was literally talking about this to someone last night on
Twitter (everyone post your twitters and we can do a Kat follow spree hihi) that Katy's LYRICS are getting more mature and are aimed at an older demographic than One of the Boys which has bratty/childishXCX'd lyrics but her persona has gotten less mature and thus her image is targeted towards younger demographics.
tl;dr - she's singing more mature themes in a less mature way which is kind of good marketing as it reaches a wider range of people