It probably also reflects a bit of a quirk when the pricing for premium connectivity was rolled out. E Musk announced that the price was $10 and then someone in Australia tweeted back that it was costing more than $10. Clearly not considering that USD10 was not AUD10. It might also seem that this did not occur to E Musk either who tweeted back that he would fix that and sure enough the price went to 10AUD per month.
So the extra 20% off by purchasing it a year at a time would be nice, but with current exchange rates you are already doing much better than the baseline US subscription.