If we're talking free tier Spotify, then it could actually be due to the bitrate (96kbps OGG vorbis, IIRC). However, if you're a premium subscriber then the standard bitrate is 160kbps, which is definitely not audible to 99.99% of people.

In fact, after much ABX testing, I found that a noticeable audible difference between a local file and the same song on a streaming service is almost always due to either a loudness differential or because the two tracks come from different masters.