The Aedra are said to be created on the concept of Statis, while Deadroth are created on the concept of Change. So why is Jyggalag, the Prince of Order, a Deadra?

Maybe it has to do with their Elven definition of "Ancestor" and "Not Ancestor". Perhaps some Anu birthed et'Ada are Deadroth and some Padomay birthed et'Ada are Aedra. It just depends on which et'Ada stayed for fled back to Aetherius, which helped to stabilize Mundus by creating the Earthbones, and which simply created their own realms to rule over (Oblivion).

Or maybe someone knows more about this and can enlighten me?

Ignore the elves, they're pompous ass turds, who are so narcissistic, that calling someone an altmer implies even more narcissistic than the actual word itself. Instead, keep the definition of aedra and daedra simple with what they did and are doing.

Aedra: The et'Ada who participated in the creation of Mundus.
Daedra: The et'Ada who didn't participate in the creation of Mundus, and reside in Oblivion.
Magna-Ge: The et'Ada who followed Magnus into the Aeritherus, and did not participate in the creation of Mundus.
Other: Those who don't belong in here. They tend to be culture gods, or mortals that ascended.

Also, all et'Ada are shades of Anu and Padomay, some just more than others. Also to note, the aedra may have been beings who represent stasis, but they caused the greatest amount of change and often do change. With the daedra, they're beings who represent change, but are very static.

In fact, Shor/Lorkhan/Shezarr is often treated as the closest et'Ada to Padomay, while the rest of the aedra are linked towards Anu. But they're all aedra, as they did create and participate in the creation of Mundus. With the daedra, Jyggy is very much Anuic, but because he didn't participate in the creation of Mundus and resides in Oblivion. Therefore, he is daedroth.

So, there is no contradiction, just mer being pompous and trying to differentiate themselves as far away as they can from humans.