Think about the verse the gospel cites; how likely is it Jesus had a seamless robe? How would they have even made it, and why? Why would it have to be seamless for the Romans to apparently not want to rip it up when gambling for it, given that Psalm 22 doesn't even mention a seamless garment? Why does only John, the newest gospel, mention it? Isn't it a bit odd that the gospel even tells you exactly how you should interpret this bizarre little incident? And Psalm 22 isn't even explicitly a prophecy, just general lamenting poetry of a condemned man. Given all this, it's pretty obvious that the author of John or his sources are making up a weird little anecdote so they can shoehorn in yet another fulfillment of scripture.