I have paid attention to the same price difference during both deposit/withdrawal, the Roobet deducted the fee from the total withdrawal amount, not from the balance. Maybe the exchange rate of the service that Roobet uses for calculating the fiat value of coins takes a commission and this causes the price difference with the average market rate.
Maybe the price difference has been caused by the market rate but the same withdrawal amount on another platform is usually higher than the withdrawal amount on Roobet. The answer is interesting by Roobet casino representative if he/she is ready to happily answer the mentioned questions.
There is a good potential chance that calculations are based on something other than what is being used, you do not always have 100% of what is going on. Just to give an example (not saying this is the reason) there could be a "estimate calculator" that would be used from one website, could be directly from mempool or whatever and it is taking from that source using an API and changes depending on whatever is going on with that situation. Whereas I feel like it could be possible for people to use direct calculation from what the blocks spend and it could be smaller or bigger for that reason.
After all, estimate is estimate but when you actually go in and send you will see how much would be fast or slow, and those two could be different. They are usually not vastly different at most few dollars, but it could still look weird to some people I suppose.