Scott Gu has a nice article describing the null coalescing operator when used with LINQ queries.
A quick example:
The Qty & Size properties for the anonymous type will be defined as integer types NOT nullable integers. If any of the spec.Attribute("x") values are null or don't exist for any reason, the value returned will be a simple integer zero.
This is because using the null coalescing operator (??) is enforcing that null can never ever happen.
Scott Gu blog article