Personally, reading the 'availability' is marginally better than reading tea leaves - certainly should not be relied upon!

In the past I've found that actual loads vs availablility depend on the time of flights, day-of-week and proportion of fully flexible ticketed pax. For example, flying US-UK on a Thursday might see an influx of the latter due to finishing meetings early and just wanting to get home - so seemingly available seats suddenly fill at the last minute - and conversely, the follwing day's flight, a seemingly full cabin empties 24hrs before it flies.