> The two most similar songs from 2007 to 2011 were Katy Perry’s “Teenage Dream” and Kesha’s “We R Who We R” , and they indeed sound alike.

I disagree. I just listened to both songs on Spotify, and they do not sound alike. Different keys, different vocal ranges, different voices, different instruments (or at least, different synthesized sounds). The Kesha song has more electronic effects on her voice. Kesha raps, while Katy doesn't. Really, the main similarity between the two songs is that they repeat the same chord progression through the whole song -- that's probably a consequence of what John Seabrook calls "track and hook".