Some people are confusing "specifications" with "requirements".
If you read the "specification" for an M4 carbine (MIL-C-70559), it describes the Colt Model 920, and nothing else. But that specification was written after the the M4 was tested, found acceptable and adopted. "Specifications" are used to ensure all items produced and procured under that specification number are exactly like the item tested, found acceptable and adopted.
Requirements are nothing more than a "wish list". say the Army wants to replace the HMMWV with a cheaper commercial truck, so they draw up 'requirements':
1) it has to go 800 on a tank of fuel
2) it has to be multifuel
3) it has to carry 1-3/4 tons of cargo
4) it has to be carried by a UH-60
5) it has to have a maximum cost of $45,000
6) maintenance costs have to be under $500 per year
And, they send this list to all the truck manufacturers, and they get back offer to sell them:
A Ford F250 with a multi-fuel engine
A Ram 2500 with a multi-fuel engine
A Chevy Silverado 2500 with a multi-fuel engine
A Toyota Dyna with a multi-fuel engine
Well, none of them will actually meet the requirements, they either are to heavy, cost too much, too short ranged, or whatever. So, they test them all and find the one that comes to closest to meeting the requirements and adopt that one. We, as civilians do this all the time without thinking twice about it (or writing lots of papers).
Then they write a "specification" that that describes the truck they picked, describes it exactly. So when the next model year comes around and the manufacturer decides to change that line of trucks to something different, the Army can point to the specifications and say, "No, this is what we tested, this is what came the closest to meeting our wish list, and this is what we want."
For purely COTS items, you don't even have to go through all that, you just state you want a Panasonic Toughbook Model CG-31, then write a quick paper on why that one and nothing else, and there you go. Sometimes the GSA comes back and grills you on your justification paper (usually at the instigation of the Senator from the other company's home state), depending o the value of the contract.