Why do I see different signatures when I create the same certificate twice?

First I do:

$ openssl genrsa -out root.key 2048

Then I do:

$ openssl req -new -key root.key -subj "C=../..."

Then I create a signed certificate in the following way:

$ openssl x509 -req -in root.csr -extfile /etc/pki/tls/openssl.cnf -extensions v3_ca -signkey root.key

If I do the last command twice, I see a different signature in the certificate. Why is that?