Good catch!
It's the tooth overhang. Nut is probably right at 4" from the center of the arbor.
You can add a dado type shim/washer between the blade set and arbor for when you use 3/8" (or both). Sucks that that will be an added step however.
If 1/4" works, the shim only needs to be greater than 1/16" (0.063")