[Ray Rogers]

Actually, they do look like tetradecahedral grains to me.
Why are they marked "octahedra"?

Ray

After thinking about my own question, I think I came up with an answer.
I will give it a try anyway.

Octahedral grain can be derived from tetradecahedral grain by continued ripening....

The grain in your picture were supposed to be octahedra...
but had there portrait taken before they were completely mature.

They were "toddlers"!