Bernard and Isana have no secrets. Bernard knows who Tavi is. That's pretty much stated in FoC. I don't have the book in front of me at the moment, but there is a part when Isana is talking to Bernard and says something like "It's happening!" or something like that. And Isana also asks Amara if "she had come for Tavi?".  He would know exactly how old Tavi really is and he probably even knows about Isana's "tampering".

But you are right, it doesn't exactly add up by a year. Maybe it's just a slip, although Jim doesn't slip on the details.