I have some bitcoins in an exchange and I found today that, without any transactions being made, the quantity has gone down.
I have contacted the exchange who have said:
the balance varies according to the prices that are floating
Is this correct? In my view the quantity of bitcoins I have should not fluctuate, only the value of said coins. Or am I missing something?
Any explanations would be great!
Only the value fluctuates, yes. If there were fewer Bitcoins someone would have to develop a fork and have everyone agree to becoming part of the fork for it to all work. As of now, no-one has tried and succeeded with that fork.
I believe that the person you quoted was referring to the market cap of Bitcoin, and not the total quantity of Bitcoin available.
Probably wallets start to pick up their spending so by users. This is wrong, I think. But you can influence.