Maybe I’m missing something, but at the bottom there are these two statements:

  7×3 ⍝ Dyadic × is multiplication
  ׯ7 ⍝ Monadic × is 'direction', or signum (the sign, in our case). Thanks.

And looking at the output I can see:

  21
  ¯1

Is it a typo or really -7 is becoming -1 for some esoteric reason that escapes my comprehension?