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.
seems by what you are saying they are less then ethical.
the only other consideration is:
and instead of say, a deposit of 1btc being recorded on their database as 1btc, they are instead recording it as the dollar value when you deposited.
which means when display that recorded dollar balance as a fluctuating btc value which changes.
this does not seem fair as most exchanges dont do this in practice.
have you read the terms or their policies that allow them to treat you bitcoin deposit automatically as a dollar balance.
did you deposit bitcoin or dollars initially
did you swap your bitcoin for dollar and are accidentally just looking at a "valuation" rather than actual balance.
it might be worth
checking their terms/policies
checking that your seeing a balance/holdings, rather than an valuation
checking when, how your initial deposit suddenly no became no longer treated as equal balance.
it may be simply that you deposited dollars and without trading have been looking at an 'estimated valuation in bitcoin' rather then a log of actual balance, mistakenly thinking they converted it to bitcoin. but actually just giving you an estimate if you traded it.
if however you deposited bitcoin but did not convert it to another currency, and they cannot explain how/where/why they have this 'floating value' feature. then i would say its unethical and scammy. and dont deposit any more with them.