I don't know about other people, but my signature does not look identical every time. It is more noticeable when some time (years) have elapsed. May be the medium used also contributes to discrepancies. If the signature is on a bottle, the surface is different than a piece of paper. Having said that, it is very difficult to figure out whether an autograph is real or a forgery. Does the vendor have good feedback over many transactions? Maybe you should send some questions and measure their knowledge. You could ask the vendor how he/she come to acquire this autographed bottle. If it seems to good to be true, maybe it is. The only autographed bottles I trust to be authentic are the ones that are signed right in front of me (as Kilian Hennessy has been kind to do).