This may not be much help, but I have found it is very easy indeed to accidentally add a pixel in PS (although your workflow suggests this has happened, given the method described I cannot for the life of me see how). I have done it myself by selecting the marquee tool, clicking in the wrong place & trying again only to find I have this additional bit way out of whack with the rest.....
Point is that it is very easy to get things wrong in PS.....and not notice.