That's right. A lot of them were ghostwritten--K.A. originally planned to write the whole series herself, but stuff happened (she had a kid, among other things), and she just wasn't able to hit all the deadlines and stuff. In those cases, she still came up with the plots and outlines and such, but other people actually wrote the books themselves.
I've started my great reread of the series--I'm reading everything in original publication order. X3 Right now, I'm halfway through number 10, and I'm getting oh-so-close to my reread of The Andalite Chronicles, which I'm really looking forward to. (And number 14, which is just so wonderfully silly; I love it so. XD)
I also found some tapes of the old TV show at my local library and put them on reserve. I'm sure it'll be even worse than I remember, hehe.