Can anyone explain how the Supreme Court parsed their two decisions?

Set aside the vax/anti-vax mandate/anti-mandate mask/anti-mask rhetoric. Plenty of other threads for that. Let's focus on the simple question of how OSHA, in which Congress has passed a law instructing them to assure that major employers have safe work sites, CANNOT mandate COVID precautions, but NIH, which Congress authorized to buy health services through Medicare/Medicaid, CAN mandate vaccines & masks. What distinction are they drawing? Or were they just looking to punt with a mixed decision? Particularly since we're only talking about a stay until the lower courts resolve the actual lawsuits involving the OSHA Mandate.