Would be interesting to check against what was the official exchange rate for the day of transactions.
I have been tracking this for a long period now, and -to my utter surprise- I found that Google typically uses a better exchange rate than the 'official' rate. For us, the final amount is within the range -1.7% to +2.3%, i.e. tolerable.
Unfortunately, I can not see a distinct pattern; sometimes its less, sometimes is more. No big deal.