Spruce boughs - bug or feature?

[Sidran]

I have one leanto for cupboard storage purposes. I am sure that I made it with elm boughs and fairly sure that I repaired it once with same boughs.
Today, when I tried to repair my leanto with same elm boughs, message said: "Spruce boughs are needed to repair this item".
I found some fir boughs from trees I planted before and repair went just fine.
Werent all boughs made equal at one moment?
Why the message doesnt say: "Fir boughs" ?