Poor quality siding will sometimes come already a little wavy. This might not be very noticeable at first. Be sure to do your product research before investing in a siding product. Vinyl siding that is very thin and flimsy is always a bad sign. Poor quality vinyl siding is very susceptible to warping, wavy appearance, and other flaws. Make sure the vinyl siding is certified by the Vinyl Siding Institute. Here is an official list of VSI certified vinyl siding products. This is an interesting video from This Old House unraveling a mystery of temperature distortion in vinyl siding.