Normally, I knit two socks at the same time on two circular needles. I find that my socks match pretty much perfectly when I do that.
However, this time around, I was trying out a new slip-stitch ribbing pattern that I'd never done before, and I thought it best to just do one sock at a time, in case I mess up. Ripping back one sock is much easier than ripping two socks back.
And, now that I'm done with the first sock, I'm really slowing down on the second sock. And somewhere along the line, I messed up my round count, so I think one sock might be a smidge longer than the 1st sock.
I think I shall have to go back to doing 2-socks at the same time.