OK I tried to follow it.

The $200 is for a change to an international trip when you have a non-refundable fare, so no you didn't get 'robbed'.

If your first fare was non-refundable also, your voucher should have been for $700 ($900 - $200 change fee). If you actually got a voucher for $900, and didnt have to pay a change fee the first time then you got lucky.