Bifur, Bofur and Bombur: From Moria but not related to Durins folk?

Question

It is mentioned in the Hobbit that Bifur, Bofur and Bombur are not descendents of Durin but are of the people of Moria dwarves.

This confuses me, since Durins folk is strongly tied to Moria:

• Balin tried to reclaim it sometime between the events of the Hobbit and the Fellowship.
• Thror too tried to reclaim it from the orcs after his people had to flee from Smaug and give up Erebor.
• Durin VI was King of the Dwarves of Moria when the Balrog was woken and killed him.

To summarize, I always assumed that Moria was a kingdom of Durins folk (like Erebor, the Iron Hills and the Blue Mountains) but it is explicitly stated that Bifor, Bofur and Bombur are not descendents of Durin.

This begs the question: Who exactly are the Moria dwarves? Which I asked here: Who were the seven forefathers of the Dwarves?.

Answer 1

From my answer to your other question, you'll see that Durin's folk were actually originally located in Mount Gundabad. Khazad-dum (Moria) was a later colony, and would have become their primary home only after Gundabad was overrun by Orcs.

The Blue Mountains were not originally inhabited by Durins folk (who only went there in later colonies during the Third Age) but rather by two different clans: the Firebeards and the Broadbeams (see Of Dwarves and Men in HoME 12) who left their cities following the ruin of Beleriand at the end of the First Age; in the Tale of Years for the Second Age, published in Return of the King, we see the following for year 40:

Many Dwarves leaving their old cities in Ered Luin go to Moria and swell its numbers.

So therefore since the early Second Age Khazad-dum had been inhabited by Firebeards and Broadbeams as well as by Longbeards.

Answer 2

Moria was also a Kingdom. It was only a colony under Lord Balin; "In 2989, there was an attempt by the Longbeards to reclaim their ancient home. The expedition was led by Balin, who had accompanied Bilbo Baggins on the Quest of Erebor. He led a colony of Dwarves there from Erebor. The Colony was successful at first," (http://tolkiengateway.net/wiki/Moria#Balin.27s_expedition)