off topic: sort problem

[Amadeus WM]

I've been using sort for years and I just ran into something I've never 
seen and I can't explain.

cat spam 
AA|3334
A|3826

cat spam | sort -k1 -t"|"              # this works!
A|3826
AA|3334


Now 

cat junk
AA|def
A|xyz

cat junk | sort -k1 -t"|"              # this doesn't
AA|def
A|xyz

The two files spam and junk have the exact same first column, but one 
gets sorted, the other doesn't. What am I missing?

Thanks!