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CK is one of many tools in your arsenal. Maybe compare to other VS3.0 TU sites like quizzle, credit sesame, or wallet hub?
One great tool on CK is they keep your reports so you can compare line by line what exactly may be different since last time. Maybe a really old account fell off? Maybe they updated payment history on an account that wasn't itemized before? Or it could just be a fluke and be back to normal next week.
It's a good sign that Discover is showing your FICO went up, I've had CK fluke before, it's no big deal. Their scores aren't generally useful except for your own gauging.
Do you have a collections or a charge-off that was recently updated as remaining delinquent?
Based on those scores, I would say there must be some sort of negative information on your accounts. You even said yourself it's a "rebuild" which suggests you're coming from worse times credit wise. As others have said, CK isn't great for scores... but even if your CK score is dropping 142 points it's because of something negative being present with the lower score that wasn't present with the higher score.
The only change listed in CK's interface; like every other monitoring solution they don't catch everything.
You'll need to manually compare the reports, most likely something in one of the negative items updated... but realistically VS 3.0 isn't really used in the credit market these days so I wouldn't worry terribly much about it.