You are comparing apples and oranges. When the more powerful original Xbox released, the weaker and harder to program for PS2 already had a substantial foothold on the market. Because of this games were developed for it then ported to the Xbox. This caused Xbox ports to looks the same as the PS2 ports with the exception of exclusives.
In this gen the 360 was weaker and less powerful and didn't have as good a market as the PS2 did in the previous scenario, however because it was easier to port from the 360 to the ps3, this made the 360 the lead platform to develop on and in most cases the quality was the same or even worse on the PS3.
Next gen will be different, the more powerful PS4 is also said to be the most easiest to develop on. It will likely be the lead platform to develop on and Xbox getting the ports.this should mean the PS4 will have the best version of a multi plat.