Last couple flights I booked had also a significant price difference. Klm.us and Airfrance.us both had different fares compared to most European websites from KL and AF.

The price difference was around 300 euro per ticket. So I always double check now. The lower tariffs didn't show on Google Flight etc.

And rarely I can find a price difference between AF and KLM, but most of the times this is a difference of a couple bucks.