That's an anti-corrosion coating.
I've seen discrepancies between sockets anywhere from HF all the way up to Snap-on.
You don't want heavy chrome inside your sockets, one, because it will modify the dimensions of the socket, and two, because the stress it is put under will cause it to flake almost immediately.
I've been wrenching for quite some time, and only in the past year or so, and only on this forum, have I seen people getting so concerned about the finish on the inside of sockets. I'm not trying to say this in a mean way or anything, just that I really don't understand the concern.
If you look at any well used socket set, the inside looks NASTY after very little use... grease, rust, dirt, etc. from bolt heads get stuck in there right off the bat.
All this says to me is that SK started using an anti-corrosion coating on the inside of their sockets at some point in the past, and this set has some older and some newer sockets in it. Almost every USA socket manufacturer uses some form of this coating nowadays.
I think what happened here, is someone posted and got all worked up about the inside of his/her sockets, and then someone else read it and got the idea that it actually mattered, and it was just a snowball effect.
SK has made a decent amount of small changes in the recent past, and I don't honestly think any SK tool you get is going to be compromised in quality. The only difference IMO is slightly different cosmetics.
If you're purchasing the tools for a showpiece/tool collection as some on this board seem to do (no offense intended to anyone whatsoever, I just feel that is the truth), maybe you should step it up to brand new Snap-on or Wright. If you're planning on actually using them on dirty fasteners, I wouldn't worry about it too much, if at all. For that use, you WILL NOT find a better USA made value for your money than the SK master sets... there is just no other option at that price range/quality level/COO, period.
What you really need to ask, is whether finish discrepancies are worth spending twice as much on Wright or Proto, or four times as much on Snap-on (to you, of course, everyone's idea of worth is different).