As others have mentioned, this really isnt an accurate indicator of actual seats available, particularly as it relates to FC upgrades. I've found that a better method is to pull up the flight in question and do a dummy booking for a high number of passengers, although I think that you can only put 9 or 10 people on the same reservation, so this is only useful up to that point. Example: if a SEA-LAX flight shows 10 seats "remaining" on the FC upgrade list but would only allow you to purchase 4 seats on a dummy booking before showing as "sold out", the smaller number is likely to be indicative of actual seats remaining.