Update runs = query execution engine processes the update plan that was generated by the query optimiser. Runs the various operators in the query plan
Value changed by the query execution engine processing the update. It's just an optimisation that SQL has not to do unnecessary work when an update does not change the value of the column
Edit: See Paul's blog post, he comes to the same conclusions I did.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability