Originally Posted by RealHJ

There will be often a small (few cents to few $) difference for exact same flights if changed/reticketed at a later date, simply due the currency exchange fluctuations. If USD has went down, the tax will go up and you will pay more. If USD has went up, the tax will go down and you will get a refund.

There also could be some government authority mandated changes in the tax rates from the original booking to the reticketing date (and for international flights that touches the origin, destination country and all transited ones).