First off, congratulations on finding this error.
I understand mint tolerances on a Wheat cent
is +-.13 grams, and yours is .4 grams. Not a huge difference but enough to get your attention..
As stated, more than likely a rolled thin planchet. That is the process for producing the flat sheets of metal from which they punch out the blanks.
Sometimes these sheets get too thin and produce underweight planchets.