Thank you both for your replies. And @franky1, that has taught me a lot!

I am with Bleutrade.

Between yesterday and today, there has been a reduction in bitcoins by 0.00214616 (not much, but it's more the principle).

I have contacted them, and they gave the reply that I originally posted ("the balance varies according to the prices that are floating").

I have been through my transaction history and there have been no changes between yesterday and today.