This lie also is the most knee-jerk in reaction of the many she told. It comes precisely at the point in the interview that LE is gently hinting that something may have happened to Caylee, that she is not "in the same state" as she was previously.
You can almost hear the light bulb turn on in her brain (similar to LA describing what LE will do when they arrive at the house and want to see if Caylee is really sleeping at Zanny's) and she instantly reacts by saying she spoke with her yesterday and immediately, as is her m.o., launches into the overly detailed book and shoe story (which sounds like it was lifted right off the "You Are My Sunshine" video), to counter the suggestion.
She obviously never had much practice thinking up things to say beforehand - if she did she would have certainly mentioned that the evening before as they were trying to locate Zanny. Sometimes I am not sure how much is indirectly because of CA's lack of follow-through and feeding her excuses, or how much may be she is just naturally the Idiot Savant of Liars.