Because presuming that all of them are produced by the same utility is a _presumption_. They could be - but they could also be produced by many different vendors using many different methods all of them simply conforming to the specification "a PDF with HIGH LEVEL DESCRIPTION OF THE DATA".
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Why are you expecting they are all very different? They're all likely very similar.
Because presuming that all of them are produced by the same utility is a _presumption_. They could be - but they could also be produced by many different vendors using many different methods all of them simply conforming to the specification "a PDF with HIGH LEVEL DESCRIPTION OF THE DATA".