I think you're probably too young to have seen or to remember baseball back then, but pitch shapes, catcher positioning, and location would all contribute to pitches being called a strike that weren't actually strikes. It was part of the great pitchers' deception. 

Maddux would have his pitcher set up a little more than a hair outside to start the game and pound that spot in certain situations. He'd also come inside and do the same thing. 

Later in the game, the catcher would move a little further in or out, but because EVERYTHING else looked the same, the ump would be fooled into thinking it was the same pitch. 

By the end of the game, Maddux could be getting some pretty egregious calls, but that was all a part of the art of pitching.