Not sure about foreign airlines, but I have never, ever seen a US airline make good on their "best fare guarantee." Caveat emptor.
Thanks for the warning. That's exactly what I'm worried about.
Airlines (or any company for that matter, will) always manage to find some fine-print as to why the guarantee is not valid in a particular scenario.
However, here it's the exact same flights... And They're offering a %10 discount on the lower fare, not just a voucher or %10 on the difference (which obviously wouldn't be that much).
Decisions, decisions.