Tax Calculation On Mobility Plans

Just Moved In

My monthly mobility bill for my wife and myself shows in this order: Plan charge, Discount for family plan, Total and Tax. 

When I check the applicable GST / PST taxes against the total they don't work out, rather I find that they are calculated on the Plan charge before discount.  Huh!  I spoke with my mobility rep about this and she confirmed that I was correct but could not give me a reason for this.  This makes no sense.  If a vendor offers you a discount on a product, then the discounted total is what you are paying and is what the tax is based on.  On my bill, Telus clearly shows the total after discount but then adds the taxes based not on that but on the pre-discounted price.  Is this even legal? 

Telus kindly provide a explanation for all of us seemingly overtaxed subscribers.