They are lying to the fans. There is no other way to look at it. If tickets remain unsold at the end of the game then, by definition, the game was not a sellout. See the dictionary definition of "sellout". Its pretty cut and dry IMO.
Now I also realize that if the FO began saying that games are not being sold out then some fans might get complacent and not rush out to buy a ticket before they "sell out". That could hurt business. I guess that fact justifies lying to the fans.
It simply needs to stop.