A lot of things can happen. Trek could have changed the graphics and coloring. The frame could have had a factory repaint. My 610 is a 1984. I know that because I bought it new from the dealer. It was repainted at the factory after a frame repair, around 1987. It came back to me painted in 1987 colors and decals. I asked for original, but they said they had no leftover paint or decals. Or a later owner could have made changed just on a whim. This is why I usually don't use the catalog pictures to decide on age. I consider them, but I depend more heavily on the serial number and physical frame features.

The components are especially variable. Old riders who started in the early '70s were notorious upgraders. later generations just used more of what teh bike came with. Many bike owners felt free to replace say a bad Shimano derailleur with an available Campagnolo part, if there were no new issues introduced by the change. The lack of "original" parts or date codes should not be taken as a sign the vintage trek site is wrong, or that you have an odd situation. You could have just had a previous owner with a wrench.