The difference is between the recording and the composition.
It is important to know that there are two types of ownership when it comes to music. There is the 'recording' / 'master' side, and the 'composition' / 'publishing' side. Traditionally this is broken up by labels (recording), and publishers (composition). More recently, things have changed, and independent creators can now handle both sides of the ownership.
Labels and distributors like Distrokid and CDBaby handle the distribution of your music, placing your music in stores like Spotify, iTunes, etc. In general, distributors will pay you royalties from your ownership of the recording. Unless you enter a publishing agreement with your distributor, you are likely not collecting your royalties from the ownership of your composition, in other words your music publishing royalties.
Songtrust is not a distributor. Songtrust is a publishing administrator. We are a 1-stop-shop for collecting on all of your publishing rights. So, using a distributor to collect from your 'recording' ownership and using Songtrust to collect from your 'composition' ownership will cover all of your royalty collection needs.