I have a Hal Leonard banjo lesson book that includes a series of tracks for downloading, including 5 tracks with tuning notes. I tuned a banjo using an electronic tuner set to 440hz, and noticed the banjo's notes didn't harmonize with the tuning note tracks. So, I used the electronic tuner to check the tuning note tracks. The notes in the tracks match the tuner when it's set to 438hz. Would they tune the banjo a little flat for the lessons? Why? Should I tune the banjo using 438hz when following the book?
A=440Hz is not a world standard tuning. Granted, a lot of the world uses it,but it's not sacrosanct. Various orchestras around the world use other datum points, for various (good) reasons, so it may be as simple as that.
Or, they may have been copied from some old recordings, which were made when 440 wasn't so important. Back in the '60s, groups would tune to whoever's instrument was already best in tune - 440 didn't really matter - and if an old piano was there to be played, that was the datum point - whatever it actually was became academic. And bands weren't so bothered about academia!
That said, it causes no harm to de-tune very slightly to A=438Hz for the lessons. It'll get you used to re-tuning and the better/quicker you can do that...