back to Order information page shipping cost increases

when am at Order information page, I choose "Purolator Ground: $34,62" as Shipping method then I go to Review Page by clicking on "continue To Review" button. after that if I want to Edit anything on Order information page I click "Go Back" button. I found that the shipping method "Purolator Ground: " has been increased to $41,15.

NOTE: even if I recalculate the shipping it will still the same. there is one way to get it back to $34,62 by choosing "pickup shipping method: $0,00" then "Go Back" it will return to "$34,62"

I think the problem in
$rates = $this->shipmentManager->calculateRates($shipment);
because $shipment has the correct value but when it calculate Rates it will came up with increased value.