Ratch did already answer the question in your previous thread. Signed multiply in two's complement and unsigned multiply are different operations, the signed multiply needs a different handling of sign extension, resulting in (slightly) different bit operations.
https://en.wikipedia.org/wiki/Two's_complement#Multiplication
Signed and unsigned addition or substraction are only different regarding the interpretation of involved bit vectors, the actual bit operation is the same.
You can review the implementation differences in synthesized hardware.