Albanian language plural deviation
My question is about a pattern in the Albanian language where the plural in some words diviates from the singular version. Something like having irregular verbs but with a uniform rule that makes them regular in their own way.
I am wondering if this pattern was created in modern Albanian or proto-albanian, illyrian or during paleo-Illyrian period.
The pattern is:
Singular. Plural.
Mashkull (Male) Meshkuj (Males)
Kalë (Horse) Kuaj (Horses)
Derë (Door) Dyer (Doors)
Lum (River) Lumejt (Rivers)
Where as the rest of Albanian words are simpler, like this
Gurë (Rock) Gurët (Rocks)
Bukë (Bread) Bukët (Breads)
And so on.
Why are these patterns different and what influenced this change?
And is this change finished, or is it still going on with more words turning into one or the other plural pattern?