You made a mistake. Sometimes once you click on an ad and the rate changes, they might send you a notification to reconfirm, which if you don't pay attention, you might not grab
Yeah, that might be what really happened.
I've this habit of manually calculating the amount I'm supposed to received for a trade. Call it paranoia or whatever you like, but I do it sometimes, especially, a big amount of money.

I feel it's possible for an exchange to experience a glitch and spits out the wrong calculation.
So, if the exchange sends a warning that the price has dropped and he didn't notice. When the exchange brings out the amount he's supposed to receive for the trade. If he calculated it manually before accepting the ad, he would be able to spot the differences and know whether to proceed with the trade or not.
I also do that very well most of the time. I mean, to manually calculate what I'm supposed to receive, it's just a way for me not to dash those vendors my money.
Before I started paying careful attention to how prices update, I almost lost money when I was having network issues. I did not notice when the price changed, and I accepted the offer and put in my selling amount. It was when I received less than what I expected that I went back and argued with the seller, and as the price difference was significant, I asked him to complete the payment or I would refund him, which I ended up doing, and my asset was released back to me.