Short answer, you shouldn’t - both systems use the same calculator.
Make sure that:
You are using the same options (receipted delivery, transport assurance, tail-lift pickup / delivery)
That the product dimensions and weight are the same as in your system (weight in kgs and dimensions in cms)
- That the Description of the packages/products being sent (e.g. CARTON, SATCHEL/BAG, TUBE, etc) are exactly the same in both comparisons.
That your origin postcode/suburb and destination postcode/suburb match exactly