I think the term you're looking for, Andre, is 'dished'.
It's down to a lack of proper quality control, during the vinyl manufacturing process and the use of inferior materials. It's as simple as that. I often encounter the same problem, even with brand new supposedly 'high-quality', 'audiophile' pressings! It does my tits in
Strangely, NONE of my (many 100s) of late 50s and early-to-mid 60s classical/jazz records are like that, and are 100% FLAT, on nice heavyweight vinyl, as standard...
I wonder why that is, eh??
Marco.