This is a longshot, but...I sing in four-part groups, and in a lot of our music, the 'lead' briefly passes from the lead to the baritone for a few measures and then goes back. When that happens, it's often notated with a dashed line angled down to the baritone line. If you have any sheet music for these pieces, that might show up...

I've sung and heard harmonies all my life, but only doing a lot of baritone work do you become aware how constantly the harmony (baritone) voice wanders above and below the lead voice within most songs. If there's a high harmony (tenor or descant) it can do the same thing, dipping down under the lead. I'm sure this is elementary to you and most of the 'pro's around here, but it helps my listening to be aware that even within one phrase a voice can move up and down across the lead, so it does no good trying to follow 'the highest' or the 'low' voice around, and maybe that's making it hard to hear the lead.

You're right, I think it's a deliberate effect to fuse the voices together. You can make the lead stand out, or you can balance the other parts so they merge. I have a feeling those family groups people think have such magic close harmonies are doing this, and the timbre of their voices matching so well makes the effect even more strong.