I was too late to edit previous post, so I'll add this:
I looked closer.
Unique unid items can have +1 more defense than perfect white items when rolled perfectly. A Herald of Zakarum would not reach 507 from a base of 168 for example. It needs 169, which white items can't have.
It's HELLA strange to see it on a shako though. If I had to guess, it would roll back down to 141 after identing because it's hard coded to max out at 141. If not, 141 is de facto NOT the max defense anymore.
I think this should be the answer:
Items with enhanced defence percentage have max Base defense plus one.
Still it's rare, not unique shako, but most likely it will have ed on it
