Generally, the FX rate at that point is just a general indication. After you submit your CC details on the website, the merchant still has to submit / process some kind of payment request on their end. Only when your CC company receives this "request" does the charge get registered and the rate finalised at that point. Might be a difference of a few hours to a few days.
I'm not sure of the correct terms, but I'm told this same thing every time I call up my CC company to check on transaction amounts.
Yes, the foreign currency conversion rates on the Internet is just an general indication of how much you can expect to get billed on your cc statement.
Based on my limited back end knowledge.. For each transaction done with your credit card, paid by E.g Visa, visa will lock in a conversion rate with the online store. Visa will then settle the payment directly with the online store, charging the online store about 3% as visa fees. (This could be one of the reasons why some of our tailors or other small businesses don't have nets or credit card machine in their store)
Visa will then bill the credit cardholder's bank, which the bank will pay on behalf of the cardholder first. The bank will then bill the cardholder whatever amount visa bills the bank, with a couple of % in bank admin fees (aka commission).
This whole process generally takes around 2 - 3 working days for most banks. That is also why sometimes you see the rate on Google is 1 Eur = 1.5 SGD, but your statement comes in to be at 1.55 SGD
That say, bank employees do enjoy staff rates too.. But that's another story.