Is this correct?

Look at my session:
http://et.splatterladder.com/?mod=sessioni...p;idx=250778695
I think I did rather good and yet I lost 0.1 rating while having 22 ppm.

and here
http://et.splatterladder.com/?mod=sessioni...p;idx=250737042
got points per minute - 15 and even gained a little bit of rating.

Whats going on?
Does it mean that I can play like noob on some servers and still get more rating than playing very good on other servers? :)

It depends on the enemies rating.
It`s not a simple PPM based calculation, it`s PPM distribution.
If you earn more PPM than the calculation algorith expected, you`ll earn positive rating.
You can find more details here: http://wiki.splatterladder.com/Rating/en