Because she starts to act more typical.
I know this has come up before. DD1 was slightly warm this morning and has been coughing. This afternoon, I noticed how much more fluid her speech was, how she seemed to be just a little sly and humorous and more animated than usual -- and looked a little funny around the eyes. Fever at 102.7, cue the ibuprofen.
Sigh. It's not a huge difference, and mostly seems to be faster processing than she normally does; but it's enough that it makes me feel like it's kind of glimpse at what she'd be like without ASD.