Voters are instructed to fully fill-in the rectangles to indicate a vote, and most do so. This voter used x-marks instead. Normally that's caught.
The ballot has marks at four races, but TEVS did not "find" the top-most mark, which, together with the other three marks, would have meant four votes in a race where the voter could vote for no more than three. Instead of discarding all votes, TEVS therefore counted the Fox, Sommer, and Corley marks as valid votes.
The approach TEVS uses is to measure the average lightness or darkness of the vote target region and, where the darkness is borderline, check the region at the center of the target region. If there's a dark pixel in the central zone (picture the central square of an overlaid tic-tac-toe board), TEVS calls it a vote. Otherwise, it is flagged as suspicious, but not called a vote. Because the voter's topmost "x" is a bit high, forming a circle in the vote target, TEVS did not read the vote.
I think I'd been thinking that by just checking the center, I wouldn't have to worry about cases where the printed dark rectangle marking the vote zone had bled into the vote region by a pixel or two. This is just a demonstration of how easily an attempt at optimizing for increased speed can let votes slip by.
I believe the county's system will have rejected all four votes as an "overvote" condition, since there is no way to interpret one of the votes as less intentional than the others, and no indication of attempts by the voter to point out a correction.