There's no evidence that would suggest a conspiracy of royals and nobles to hide the fact that their better stats are acquirable by anyone. For one part, Stanley is a non-royal and would thus know if it was true, as he'd know if he got better stats from becoming an overlord. And would have mentioned this at some point, so Sizemore would have explained it to Parson. So, no conspiracy. Royals and Nobles are better, statwise, than regular units.
A royal does not have automatically better stats than every non-royal for their level, their is some randomness involved, so I'm assuming that is just an average. If this was not the case Jillian's stats would have been to high for her level, and Ansom would have immediately known she was a royal/noble.

Also since promotions can up unit stats (making a unit a warlord adds leader ship) it is possible making someone a heir gives a stat boost that is equal or bigger than, the royal bonus, and if it doesn't stack the royal bonus can be gained by anyone if the ruler pays excessive amounts of money.