The formula for the rates we offer is fairly complex: it's dependent not only on current prices but also liquidity volumes available along the buying paths open to us for replacing the coins we're selling, our available inventory, and even our long term view of the coin's potential.
To clarify the implications of the above description, if you want to buy or sell a lot of a coin (a lot here being defined as more than what's available around the immediate spread price on exchanges that offer the coins in question), then you'll likely pay more (because we will have to pay more if/when we replenish the coin we've sold). This also protects us from using a fake "low-liquidity" price that someone might try to spoof on one of our pricing sources. So if you got estimates for the same transactions as those you did above, but sending a smaller amount, you might get a better percentage price (on the other hand, it could be worse if you go too small, see next paragraph about transaction fees). One way to get a better rate from us is to place larger orders when the overall market is "hot" and has a lot of liquidity available (but high liquidity is often associated with price volatility, so that's the counterpoint to that view).
Another factor that comes into play is the transaction fee for sending the coin (we pass this cost on to the buyer). But this mostly has impacts on smaller transactions and is very coin-dependent. For example, if you bought $1 worth of btc, you might be paying more for us to send it than you would be receiving because of the cost for us to send it to you.