The 7-way RV plug (flat pins) had a couple of conflicting standards over the last few decades. Its easy to assume the way it is now is the way it has always been, but that's not the case. There is what is called the 'traditional standard' and there is the more recent 'SAE J2863' standard.
Then there is the round pin 7-way plug that has it's own standard, different from the other two.
And you have not mentioned what kind, brand, or age of trailer you are working on. Could be that you have a mix of cargo trailers and a camper trailer. (sure, we can make assumptions but those assumptions often turn out to be wrong)
Whatever type and age of trailer you are working on might be decades old, or it might have a pigtail that is wired for the older standard...maybe a previous owner re-wired it for the other 'standard'.
So it turns out that it IS rocket science...sort of. You have to be smarter than the plug you are working on.
Here is more info: