Why some streaming hi res versions are lower SPL

The answer to your question is, every recording or recording studio is setup different has different people the sound is going to be different good or bad and the source clearly shows this. If your system is able.

The answer to your question is......Who knows?   That's just the way the recordings were recorded/engineered in the recording studios.  I recall how there were variations in volume/dynamics that would occur, sometime, from track to track on the same CD, SACD or DVD Audio disc.  It's the same for streamed music, as well.  But, you're absolutely correct in your observation.  I noticed what you described when listening to streamed hi rez classical/orchestral music to most.   Happy listening.