The designer says,

I like to think of this Scarf as the ‘Ugly Duckling Scarf’.

It’s based on the same pattern but using a different color scheme than any of the previous scarves, it’s at least ten stitches wider than any of the previous scarves and for some reason, the production staff went back to using a shrunk up Season Thirteen rather than continue with this scarf.

I have yet to hear an explanation for why, and the moment I do, I’ll of course share it with all of you!

The Season Fifteen was seen in only three episodes: Image of the Fendahl, Underworld and The Invasion of Time.

After that, it completely disappears.