Thanks @exyle. After I'd posted this I remembered what you told me about the relative price of Bitcoin and Steem and I thought it might be a similar principle. However, until recently we weren't being paid in Steem were we? It was just SBD and SP.
Or in this case is SP the same as Steem except one is liquid and the other isn't?
I think I might be getting it. Slowly, slowly. 😁