From: Brighton, UK
"New Releases" is an umbrella section for all games that are available, which can be 'released' games (i.e., version 1.0 or greater) or 'early access' ones, like SPM. Notice that SPM has been under 'New Releases' since the Early Access Program started last year, nothing changed (well, it got pushed down the list as new games were released afterwards ).
The Early Access Program is no longer available for purchase, but development on the game continues. SPM version 1.0 will be released on October 31st, 2014.