PKCS#7 Signed Data contains always different signature
That's likely because the data used for the signature generation actually changes between the signing. That would e.g. be the case if that data contains the signing time. Note that within CMS the data is not directly signed; it is first hashed, put in a structure and then that's what is getting signed.
Also note that inclusion of the local time generated by the system is not the same thing as using a time stamping service.