Can Openers Again

This appears to be the WWII era patent for the P-38 can opener.  However, it appears that there were a series of earlier patents for can openers that were pretty darn similar, if not identical.  The earliest seems to be from 1913.  One question that pops to mind, is how could the latter of these patents have been allowed?  My guess is that 1) maybe the examiner for the latter patents didn't know about the existence of the earlier ones, or 2) each of the the latter ones have slight changes to the design, and the claims include the changes.  The changes don't have to be improvements, they just have to be different than the earlier design, and not obvious.  Thanks to my buddy Eric for the in-depth search.

Another post on the P-38 can opener.