If I issue select pk from MyTable where MyNonUniqueColumn = 123

the execution plan shows it uses an Index Seek on MyNonUniqueColumn index (is good)

if instead I issue select pk,field2,field3,etc,etc. from MyTable where MyNonUniqueColumn = 123

the execution plan now shows it's using a Clustered Index Scan on my PK index!

What's going on here?