Hi Livia!
Not sure if you got an answer on this but I do believe your suspicion is right with the changing exchange rate on a daily basis.
On Fiverr Help, I found this:
All prices that appear on Fiverr are based on their original US$ price and, as a result, non-US$ currency prices may change on a daily basis, in accordance with exchange rate fluctuations.
I assume this applies to payouts as well! You can read it here:
https://help.fiverr.com/hc/en-us/articles/360050217393-Using-your-local-currency-on-Fiverr
I am in Canada too, and typically just work in USD on my profile to keep it easy to track and whatever the exchange is when I withdraw is what it is, but you can always monitor it!
There is more about withdrawing earnings here as well:
https://help.fiverr.com/hc/en-us/articles/360010530058-Withdraw-your-earnings
Have a wonderful holiday season!
Chelsea