Are the terms “reasonableness” and “due diligence” ambiguous?

In Ergon-West Virginia, Inc. v. Dynegy Marketing & Trade, the Fifth Circuit found that Dynegy had no duty under two natural gas supply contracts to attempt to get replacement gas after a declaration of force majeure in response to hurricane damage, affirming the district court as to one contract and reversing as to the other.  No. 11-60492 (Jan. 22, 2013).  The first contract’s force majeure clause required Dynegy to “remed[y] with all reasonable dispatch” the event.  The Court found that “reasonable” was not ambiguous but that extrinsic evidence of industry standards (favorable to Dynegy) was properly admitted to give it full meaning (contrasting its approach with the district court’s, which found the term ambiguous and admitted the testimony to resolve the ambiguity).  The second contract’s provision had language about “due diligence” by Dynegy.  The Court found the term ambiguous as both parties’ readings of it were reasonable, and then held that the district court should have credited the same evidence here as it did for the first contract.