That holographic window is a bit of a double-edged sword. One the one hand it is the easiest place to show damage, including from the distribution process, before the note even enters circulation. And on the other hand, any blemishes here devalue the note pretty quickly.
As a casual collector I didn't know this at first, or that the TPGs (or at least the good TPGs) inspect notes under magnification. That's when I started scanning my notes and zooming in on all parts, most especially the holographic window. I once received a nice run of crisp UNC notes from an ATM that all had similar tiny scratches when viewed under magnification. Needless to say, they all went back into my wallet as spenders.
And I'll echo what's already been said here, that BCS is the TPG I trust the most. Mango4's grading results are definitely worth being proud of.