Why gpg-signed files differ every time

I signed the same file producing a separate files for the signatures, and I noticed that every files is signed with a different signature. I don't get it. Shouldn't it always be signed with the same public key in the end?

gpg2 --output FILE1.sig --detach-sig --armor FILE I made 3 files (FILE1,2,3)

every file has a different signature - why is that? Please explain!