Many clans have branches (away from the clan lands) where they are septs of other clans.  Perhaps this is why ABERNETHY is both a clan & a sept of another clan.  Scots Kith & Kin shows clan LESLIE only has 2 septs -- ABERNETHY and MORE -- so ABERNETHYs should get a lot of attention & a big welcome in Clan LESLIE.  http://www.tartans.com shows ABERNETHY as a sept of LESLIE & FRASER.  Scots Kith & Kin confirms that ABERNETHY is a sept of FRASER, but FRASER has about 20 different surnames as septs so Clan LESLIE would be a better home for ABERNETHYs than Clan FRASER.  If you knew your ABERNETHYs were from the area of Clan LESLIE, you would join Clan LESLIE & if you knew your ABERNETHYs were from the area of Clan FRASER, you would join Clan FRASER.   If you did not know where your ABERNETHYs were from, it would be best to join Clan ABERNETHY.

ABERNETHY is perhaps a better clan to claim than ROBERTSON since ROBERTS is probably a surname that was invented in both England & Scotland since Robert is a given name in both countries & ROBERTS is short for ROBERTSON -- "son of Robert".  On the other hand, ABERNETHY is probably a surname invented only in Scotland.  This means if you participate as a ROBERTS in Clan ROBERTSON you may be celebrating a connection to Scotland that is not yours.  Whereas with ABERNETHY, your ABERNETHYs have to be Scots since all ABERNETHYs are Scots.  At least that is what I suppose with what little info I have on hand.  I don't think anyone is doing clan tents for Clan ABERNETHY so the field is wide open to start up the clan & do it your own way.  Your ABERNETHY ancestors need someone to champion their clan.  ROBERTSONs on the other hand have plenty of people championing that clan.

Surnames of Scotland by George F. Black says the "origin of the Abernethies cannot be stated with any certainty."  It "would seem ... that they were descended from original native stock and not of Saxon or Norman origin."