According to this answer,
- Month reputation is the amount by which your reputation changed that month, excluding "private" reputation events. Certain reputation events are excluded from your public user profile, such as deleted posts or downvotes,
This user seems to have (had) a high scoring answer, because they have a "Good Answer" badge for an answer which has now been deleted. Those 250+ points are included in their monthly reputation but not in their total reputation.
Nice puzzle :)