I think I read somewhere that sudo does not pass along all environment variables on purpose, since that could be used to introduce root exploits (not unlike the recent bash bugs). Or you may need to use sudo with -E

 -E, --preserve-env

             Indicates to the security policy that the user wishes to pre‐

             serve their existing environment variables.  The security

             policy may return an error if the user does not have permis‐

             sion to preserve the environment.

So you may have a different "security policy" set, somewhere. man sudo has more info, it appears to have more settings than just su does.