Manly referring to songs on this post for this answer (oldies/Classic hits): If it's live and there is not talking between songs, the jock can turn the songs volume up, but it is amazing how many older songs begin to fade 25-30 seconds before they end. If it's running on Symbian , etc., those systems have a warning signal that triggers the next song and usually isn't programmed closely enough to catch the lyrics at the end of a line, etc. I think it's more a sense of play it safe and not allow dreaded dead air. Waiting too long to start the next song can often make it sound choppy and faded (I call it anemic), so yeah, it's a fine line on just how long to wait. Plus, the true old Top 40 format was always tight and part of the art was the quick cut away from one song at the exact best part of the ending lyric or beat and cranking into the next song. Top of the hour stuff was always fun. Gotta roll into the new our with a jamming tune. Firepoint - I will save my theory for turntable radio for later.
But most listeners (including me!) hate it whenever someone "butchers" an ending by prematurely going into the next song. At the top of the hour, I tried to time it out so that it would hit right with the network news, or else I played something with a lengthy fade, so that listeners wouldn't really notice if I had to back out of it a bit early.