[Linganth] Substitute x for y

[Ronald Kephart]

All:

More than twice now, I have gone to our local fried chicken outlet
and ordered the 8 or 12 piece dinner, which usually includes some
breast, thighs, legs, and wings. While doing so, I have said:

"Please substitute legs and thighs for the wings."

And nearly every time, the response I get is:

"So, you want all wings?"

I cannot understand how they can interpret my request in this way.
For me, "substitute x for y" should mean that I don't want y, I want
x instead. But my wife who seems normal otherwise, agrees with the
chicken people. What's going on here?

Ron