Technically, glass is flexible. It's rarely perfectly flat. Naturally, this can be more pronounced with thinner glass. I think many advanced window systems do use slightly thinner glass than a typical home window.
The actual flex of the glass may be very subtle, but the visual effect is amplified by distance, in the same way that small defect in a lens would be unnoticeable in a desk magnifying glass, but would completely ruin a telephoto camera lens.