I actually noticed this as well, I did some investigation and it appears that you get your deposits back using the same exchange rate used to originally deposit. However, anything above this amount uses a slightly lower exchange rate which I do find a bit dodgy, especially as they only ever allow you to convert your local currency into US Dollars without the option to instead use your local currency.
I withdrew $200 in total.
Credit back was for £41.34 (this is the amount i'd deposited), this was based on $52.83. This equates to an exchange rate of £0.792 to $1.
The remainder was sent as £115.11, this was based on the remaining $147.17 of the $200 total. This equates to an exchange rate of £0.782 to $1.
The difference is minimal but it shouldn't really exist in the first place. I don't doubt the legitimacy of Pokerstars and their intentions, they are a great business, but I do find this practice a bit shady. I'll also add that my withdrawals have never taken more than 24 hours (playing at Pokerstars UK).