Clifford inlet manifold compatability

https://www.thezstore.com/page/TZS/PROD/classic17a07b/11-2995

 

For Use With Our Headers:
If you are going to run this Clifford manifold with one of our headers, there will be a small area of interfere between them. Slight modification (grinding) is required to the mounting flange to allow proper clearance. The small amount removed will have no adverse effects on the install, the manifold, or the header. 

For Use With Other Headers: The manifold will NOT work at all with Nissan Competition Headers, or Hooker headers. 

 

Can anyone explain to me please why a performance exhaust header won't work with these ?