The old PNG specification also explained the rationale: http://www.libpng.org/pub/png/spec/1.2/PNG-Rationale.html#R....
But the new spec doesn't explain: https://www.w3.org/TR/2003/REC-PNG-20031110/
But the new spec doesn't explain: https://www.w3.org/TR/2003/REC-PNG-20031110/
On the one hand I sort of understand why they don't "If it is not critical and load-bearing to the standard. Why is it in there? it is just noise that will confuse the issue."
On the other hand, it can provide very important clues as to the why of the standard, not just the what. While the standards authors understood why they did things the way they did, many years later when we read it often we are left with more questions than answers.