I don’t think it’s just the FF sleeves. The bottom of the sleeve has a seam that creates a thicker and/or stiffer side than the top where the opening is. This has less of an effect the thicker the sleeve is overall, especially if the side seams are as bad as the bottom seam.
That’s essentially what I end up doing, but with Terraforming Mars, there are so many cards that they don’t slide all that easily and you have to do it so many times…