Come on you bunch of amateurs!! Give the guy some proper advice!!

It depends on what type of probe is used. If it's a proper Exhaust Gas Analyser, where the probe goes fairly deep into the tailpipe and you are seeing Lambda 2.0 then there is a problem.

However, if it isn't it could be as simple as a draft into the tailpipe affecting the reading. Especially if this is on a Dyno with the fan running.

An exhaust leak would also cause this.