Amazon- currency converter vs. XE vs. BOI mastercard

I ordered some items on amazon on 30 april. Came to

-Order Total: GBP 93.48

using their currency converter it was

-Payment Total: EUR 112,43

I checked on XE at the time and it was

93.48 GBP=107.672 EUR

Of course nobody gets this and it is usually 2-3% more than this using CC or paypal so I chose to pay in sterling via CC.

My BOI mastercard statement was
93.48GBP RATE 0.8549 €109.34

So mastercard was only 1.5% more than XE
Amazon was 4.4% more
And I saved €3.09 by choosing to pay in sterling rather than use amazons converter.

My statement shows the payment on 4 May, so I wonder if it is taken on that date and if the XE rate would have been the usual 2-3% I would expect.