Why is Spotify using so much data playing solely songs I have downloaded to my phone?

[Colin5]

Why is Spotify using so much data playing solely songs I have downloaded to my Android phone? Yesterday it used 21MB on two 90 minute journeys, 18MB foreground and 3MB background. I was listening to a playlist of my own, all downloaded to my phone. I was deleting some songs and moving some songs to another of my own playlists, also of downloaded songs. It wasn't in offline mode, but why would it be using data so excessively. This is not sustainable within my phone's monthly data allowance and I ran out of data last month because of this.