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.