I think almost every site has an instant deposit as you transfer your coin, it gets delivered after confirmation but for withdrawal, I think the site will take a little time to check on your withdrawal request and processing.
Deposit credited to our account after confirmation is not something called by instant deposit. As per my understanding, instant deposit is where you do not need to wait for confirmation of our deposit. Not many sites offers this feature, although there were many sites had this feature but they decided to use normal deposit (wait for confirmation) because there were many double spend cases which made them lost some money.
Many thank you for the explanation. My deposit of 200 mbtc was ready to bet about 10 seconds after i sent it from my Blockchain wallet.