If you received a $10 reward, then you spent between $50 and $99.99
If you received a $20 reward, then you could have spent $100, $125, or $149.99.
If your purchase is $98, then you'd be better off spending $2 more, so that you'll get another $10 gift card. That would be a net gain of $8, plus the other $10 gift card you got for the first $50 you spent.