Writing a series is very hard to do. And when you have one go one for this long it is extremely difficult to have perfect continuity even if everything is plotted out in advance down to the nth degree. Things constantly crop up that you never thought about or don’t work out the way you’d planned or the way you planed it puts you the author in an unexpected box with no way out.
Professional editors help, which IIRC, Mr Conroe did not have when he started and maybe still doesn’t, I don’t know for sure. But even they can’t catch everything.
That being said, there are several continuity issues in the series, this is just the latest one.
I still like the series though.