The telecine transfer of a given movie can be a little off, yes. But I've only seen SLIGHT variations, nothing major.

My 16:9 TV is carefully tweaked for horizontal/vertical size and alignment using a DVD called "AVIA" which has test patterns for the exact anamorphic ratio and overscan marks.

Once I got those test patterns looking right, the subtle variations in ratio from movie to movie became not-noticeable.

However, I know that there are some TVs and some DVD players that try to auto-correct certain things. For instance, I think certain Panasonic players will auto-stretch a letterbox (non anamorphic widescreen) to properly fill a 16:9 screen set to anamorphic mode. Dunno if that's what's happening.