When I joined Vodafone I was given a special offer on my iPhone 24 month contract, $20 discount per month.

I just looked back to when it started - December 2014 and it says every month since then:

$20 Exclusive Discount 13 Dec - 12 Jan 1 cr $17.39 etc. Why has my $20 discount been only $17.39? - I wondered if there was a logical reason or if anyone else has the same discount that maybe it is gst related or something?

Would I be right that Vodafone owe me 14 months x $2.61 making $36.54?

Thanks.