Why some streaming hi res versions are lower SPL

“The best hi-res releases keep the original dynamics while adding clarity.”

Totally concur. If they don’t sound as such, I try to play 16/44.1 version. 

It mostly comes down to mastering. Some hi-res versions use a different master than the CD, which can affect volume and dynamics. If the hi-res version has less compression or peak limiting, it might sound quieter but more natural. EQ differences can also change the perceived impact. Some DACs handle hi-res files differently, which can affect playback. And sometimes, expectations play a role—if a hi-res version is just cleaner rather than dramatically different, it might seem less engaging. The best hi-res releases keep the original dynamics while adding clarity.

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.