You have hit on a common programming philosophy there, and it is driven by the realities of timing.
At the end of the hour (which is what we more commonly refer to that time span as; to us, "bottom of the hour" is :25-:35), we might get one song, two songs, or three songs in between the last commercial stopset and the top of the hour (that term, you got spot on!).
We can always take for granted that the first song is going to play, absent any unforeseen glitches in the playback automation. At night, we will often presume the second one always will. The rest are going to be dropped by the automation when it hits the TOH real-time reset, and we don't want those to be critical to the overall format integrity, so those will be programmed just as you noticed ... recurrents and neutral-testing gold titles.
Good observation!