Which makes perfect sense when you look at it the way Okuda and Sternback did in the TNG Tech Manual. Their brilliant idea was that messages travel in subspace at extremely high speeds - but only for a short distance. So a message sent by a ship outside the reach of a buoy network does not travel more slowly as such, it just travels fast until a certain point, and then crawls to the nearest relay buoy.
Different ship transmitters would have different range, translating to different delays in messaging. And with small craft, it could be that sending a message to Starfleet might take aeons, but receiving one from Starfleet might be quick, because even the relay buoys have longer-ranged transceivers! This would account for certain DS9 anomalies with runabout communications, for example.