Bare in mind we also interpolate, we can't query every amount, after all. It also wouldn't be a good design for user privacy to send a request for a quote every time a user searches for a pair, because if any of the exchanges we list had their query history compromised it would make it possible to timing-attack our users.
That makes sense. But still, dkbit98's example showed a 4.5% difference. It makes me wonder how often you check those rates. According to CMC, XMR went up 0.91% in the past hour. If you checked more than an hour ago, your estimate could be almost 1% off already. Which makes me wonder if you check different exchanges at the same time per trading pair. If you checked XMR-BTC on exchange A an hour ago, and check it on exchange B now, there's a 1% difference already and you may show the wrong one for the best rate.