If it's a part of an "official" system/package from a big defense contractor there's little chance it's counterfeit. There's quite of bit of scrutiny there, unless the contractor providing that system is buying these off Amazon which seems the lest likely scenario. But maybe a small company setup with the sole purpose of winning the bid for an F-15 simulator bids so low that they do buy this stuff, it's possible.
As others mentioned, there are layers of rules about justification if you don't go for the lowest price, preference for small business, and getting bids.
Finally, if you just need something, the person ordering the item isn't the person buying the item. The IT professional orders a Cisco router, provides some possible places to source it and turns that into contracting/procurement. The procurement team will then actually go buy it and often they will go try to find the cheapest option and they're not experts so they'll often buy the wrong damn thing entirely. Want a 500ft roll of yellow shielded Cat6? Here's 1000ft of blue UTP Cat3 because "it was cheaper".