That's because DW had Danielle in there later--and Brenda wouldn't know that.
The only other comment I have (and I'm sure your thrilled--sorry!), is each and every time Westerfield talks about there supposed to have been a babysitter at the van Dams, he has this tone of wonder and surprise in his voice. He says it first in the media interview that was shown to the jury. He says it a few times in his interviews with detectives. He kind of pauses of muses aloud, sometimes phrasing it "I could have sworn she said..." that it isn't just stated matter of factly in answer to a question of what did Brenda say.
The babysitter comment was in that post as it was in the affidavit and the docs confirmed it was not said out of the blue to the detectives prior.
As for DW's tone of wonder and surprise in subsequent conversations....she did tell him she was going to get a babysitter...and would be at Dad's if she did....she was.