The mass backing may be the problem. The original 60s-era OEM carpets had jute backing.
That hard plastic stuff may have distorted the contour of the actual carpet.
I've never had a fitment problem with ACC, which I've used on many cars, but it's been a few years since the last one.
I generally ended up trimming the sides, and occasionally the firewall area.
I'm wondering if someone had the bright idea of supplying less material to reduce costs, and it's not exactly figured out...