spotify is using cellular when on wifi

It may not be the only app doing this but is one I am sure of. I tested it by

1. resetting cellular statistics

2. turning off cellular for Spotify

3. launching Spotify (I received that silly warning informing me that cellular was off - I know that ... I turned it off - and I don't need it because I am on wifi)

4. played Spotify for a bit.

5. checked cellular usage - it had not changed.

6. turned cellular back on for Spotify

7. launched and played Spotify for a bit

8. checked cellular usage and it was now higher for Spotify

I've seen others reporting this issue around the web but have not seen an answer yet. Thought this might be the best place to get one.

this is on an iPhone 6 using iOS 8.3

ps: I noticed some system services also use cellular when on wifi. Not much usage ... but it's there.