TalkChess.com

[Kevin Hearn]

Just looks like the difference between fail soft and fail hard. Because you may have made different pruning decisions with a different search window when you fail you return the hard bound because you're not sure if you would've found something in the range (beta, score) in a future search when you look at branches that were subject to pruning with a lower window.

Shouldn't be anything special about multithreading that changes which way you decide to do things, both have their points.