A320 inflight engine start
 

The highest altitude for an stabilzed inflight windmill engine start for the Leap 1A is FL270 and FL250 for the CFM 56. Does the top of this envelope simply mean that attempting an engine relight above this altitude is less likely of being successful? The all engine failure QRH procedure says “approaching or below FL270” (Leap 1A), attempt a restart. Is there any reason why not to attempt an engine restart above the engine relight envelope?