Why would sp_helprolemember show different MemberSID for same member?
dbo is a user in a DB. When a user is created (whatever is name would be), it gets its own unique SID. If you later create a second DB and add a new user with a similar name (or not), it will get another unique SID. Both user on DB1 and DB2 have nothing in common and SID will be different.


  This was based on the unedited question about sp_helpsrvrolemember...


Look at the view sys.server_principals which is mentionned in the text below from the technet link:

When a SQL Server login is created, it is assigned an ID and a SID. These are visible in the sys.server_principals catalog view as principal_id and SID. The ID (principal_id) identifies the login as a securable within the server. It is assigned by SQL Server when the login is created. When a login is deleted, its ID number is recycled. The SID identifies the security context of the login and is unique within the server instance. The source of the SID depends on how the login is created. If the login is created from a Windows user or group, it is given the Windows SID of the source principal; the Windows SID is unique within the domain. If the SQL Server login is created from a certificate or asymmetric key, it is assigned a SID derived from the SHA-1 hash of the public key. If the login is created as a legacy-style SQL Server login that requires a password, the server will generate a SID. 

https://technet.microsoft.com/en-us/library/ms403629%28v=sql.105%29.aspx