Some stores work on a monthly average, while some others work on a daily one.
Rates go up and down all the time, but usually not by very much. It can be onerous to track each change, and most stores prefer to deal in close to round numbers when it comes to pricing.
Amazon for instance, change prices constantly, certainly for ebooks, which I monitor. They do it at least daily, and often it is either up a cent or two or down a cent or two, and rarely more than 10 cents. Kobo on the other hand have set prices for ebooks, that rarely change, unless a big currency change has happened.
Most of the time the majority are minor differences and come out in the wash ... balance out. So a store might gain a little benefit, but they are just as likely to get a little loss too.