Imagine 4 brothers, ones name is Chebby, another's name is Oldman, one's name is Poncho, and the last guy's name is Buck
... Mr. GM, their dad, came up to them and said, "I want each of you to design an engine".
so they all went off into their own little rooms and made V8s. Non of them knew what the other was doing, nor did anyone go into each other's room to see what might be happening.
The result was 4 VERY different 350s. They are all V8s, and all essentially have relatively the same displacement. But they are all different in everyway.
Chebby, Oldman, Phonco, and Buck later went onto design many more different engines, big blocks, small blocks, V6s, etc...
Also, Mr. GM noticed that Oldman, Buck, and Poncho all design the same bolt locations for their transmission. This is known as the BOP transmission bolt pattern. So poor Chebby couldn't use his transmissions on Oldman, Buck, or Poncho's engines.
... true story