what the difference between "If a matching ExternalURL setting isn't found" and "If no matching ExternalURL setting is found"?

http://technet.microsoft.com/en-us/library/bb310763(v=exchg.141).aspx

for the above article at the section "Proxying and Redirection for Outlook Web App"

3 .....

"If a matching ExternalURL setting isn't found, CAS-01 will look ..."

"If no matching ExternalURL setting is found, CAS-01 will look ..."

it seems the above sentences express the same meaning for a condition,so it doesn't make sense..so it seems the article want to express the meaning : if the

ExternalURLproperty hasn't been configured  instead of "If no matchingExternalURL setting is found"  ,right?

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