Mid-80's To Early 90's Lincoln Cent Reverse

Going through a pile of cents I notice a trend and I wonder what people here think of it. Based on my observations it must be pretty common.

LC coins minted from 1985 - 1992 that I examined (I did a marathon where I looked at 300+) seem to have an inordinate number of issues in the area of "S oF" in the UNITED STATES oF AMERICA on the reverse. The lower half of the letters seem to be flattened/weakly struck. I have one coin (1988D) where the word "oF" is almost completely missing. I put so many of them aside that I just stopped counting how may from that date range had the error. It isn't wear because the letters on either side of the ones I mentioned are fine. Attached is a pretty decent picture of the 1988D.

Is this something that others have noticed? I was figuring they are die grease errors. I'd love to hear what people have to say.