This is not a slip-up. The cover pictures don't match the book. You can see Jake morphing in really big baggy jeans, and yet they stay on him during the morph.
Yeah, the covers don't match the books. For ages the covers of tobias' books had him with brown hair even though the book said it was 'dirty blonde'. Later on in the series, his hair colour changed to blonde...
Right. It isn't the author herself who makes the covers, you know. He (David B. Mattingly) is an artist, so he makes what fits best. So this isn't a slip-up.