Document -eq operator when right and left operators are lists about powershell-docs HOT 1 CLOSED

Short answer is that we don't really support comparing two collections using the comparison operators.

Long answer: the right-hand operand is treated as a singleton. That singleton object (array in your example) is compared to each item in the collection on the left-hand side. Since the singleton object is an array, the comparison becomes a reference comparison. It will only match when the right-hand object refers to the same instance of the left-hand object. Since you are comparing $a to $a, the -eq operator will find matching references.

When you compare $a -eq "abc","def", those are two different instances, so the references don't match.

If you want to compare all the values in one array with the values in another array you have to enumerate the items yourself and compare.

from powershell-docs.