Hopefully mswood will get a chance to respond, but she did post a few weeks ago that Nielsen has changed how the count audiences, and favors multiple repeat viewings more than it has previously.
The entire CBS lineup has lower numbers, but it still is in first place among the broadcast networks on Sunday nights.
And there's also the increased viewer options on Sunday nights. HBO, Starz, Showtime, SyFy, you name it are all airing Sunday night dramas on cable.
The other factor is that there has, in fact, been fewer sports overruns to deal with this spring.
I'll grab and post the final CBS press release for the week when I get more time. It's been extremely hot here the past few days, and humid, and this is weather that hasn't been seen here, literally, in 100 years this early.