We're the reader, it's the tool.
I personally follow what I consider fairly traditional meanings and give each card few choices. But the layout, how we decide which card will mean what, and there are differences of opinions there as well, even among traditionalists, there are different schools of thought, literally, well, how we read layout matters obviously.
As far as "rules", even among traditionalists, there are some variations. Although I found the meanings of each individual card, for the most part, fairly similar among different schools, different readers, it was how a reader read the spread, which card stood for what, where the differences showed up more.
I think those who are somewhat traditional readers, even if they may not read EXACTLY the same, can understand how another traditional reader may have arrived at their reading.
If someone is totally an intuitive reader, and I am not saying anything pro or con about that and opening up that can of worms, but is someone totally marches to their drummer, I would think another reader who reads more traditionally, may have a problem seeing how they got to their reading.
As to why some people don't follow "rules", sometimes they may, just not the ones we follow or sometimes because they feel they do better reading intuitively. I really don't know, all readers are different.
To me, Lenormand is more about the deck, than EXACTLY how one may read it because there is more than one set of "rules" or schools. I like having a structure to the cards, I think they read like a book, others may not or follow a different structure.
In the end, I only care how I read and if my sitters feel they get something of value from the reading.