Not wishing to contradict you, Peter in the historical naming of the tartan in question, but one reason I think that there was a confusion between which of the two setts should belong to which clan is that both were in possession of the same tartan, sharing a close lineage, Glengarry sometimes even referred to as Clanranald of Glengarry in those earlier days. Anyhow, the difference is now well established.

Clanranald

Glengarry

Images and names provided from the Patterns of the Highland Clans on the Scottish Tartan Museum website.