Hedge

What, exactly, is a slight change in venue? Getting people to move from the left side of the room to the right, perhaps? (No, it was a complete change, from one lecture theatre to another.)

The use of slight above is an example of a hedge, i.e. ‘a mitigating device used to lessen the impact of an utterance’.  Using it probably made the writer feel there was no need for an apology.