As far as I'm concerned Family Search data come from parish registers etc, and you can see the film number they've quoted meaning that it comes from a filmed copy of a register, bishop's transcript etc. But I have seen a lot of these discrepancies in baptism dates recently, and maybe there's some pattern to them.
The Mormons filmed a great number of registers long ago in the pre Internet era, and humans transcribed the content into the computer format of the day, as part of the original IGI, but I suspect the data have been converted quite a few times to end up on FSearch, and haven't been rechecked against the originals. You also get wrong locations showing sometimes, probably for the same reason.
