I have also been keying the latest date. I think sometimes there are typos, but a lot of times people retired after 20 years of service and then reenlisted during WWII. All of the ones I have seen where there are dates after the date retired have been WWII era.
It's also possible that people retired from active duty and stayed in the reserves and were called back to active duty during the war. There are some "dates of temporary rank" where they would have been given a military rank and then gone back to their previous rank in the reserves after the war.