I would say it was all in the matter of the producer and the engineer keeping the tape running. Usually they would only keep two takes of a vocal performance, the master take and an alternate. They would extract those,erase the other takes and use the reel for the next session. For the most part that stuff is on the multi track tapes which is what the final mixes are made from. When we hear that stuff it's primarily because mixing was involved. Kevin Reeves and Co. are listening to the reels so they stumble across it. Harry Weinger who's big on the "extras" leaves it in as a treat for fans. The "Master" tapes just consists of the song[[s), the "Multi Track" tape is where everything is in totality and in raw form. Hope I helped a little.