What Howard said, and I'll add:
What would happen if the naval base at Bangor was suddenly buried in lava, then dug up two thousand years later by archeologists interested in how the American military functioned during the twenty first century?
We have at least two branches always there (navy and marines) and most likely, there are folks there from the other three branches at any given time as well.
Not counting various dress uniforms, the marines will be wearing two different colors of MARPAT and there are also the old BDU's still in circulation, both woodland and desert, and two different styles of boots.
The navy? I won't go into how many non-working uniforms there are for various occasions. For working uniforms, you'll see either working blues or whites, utilities, coveralls (both blue and green), BDU's, two different styles of jackets for the BDU's, and five different jackets for utilities/coveralls/blues/whites, depending on the command, the location, and just what the individual felt like wearing that day. Two different hats for blues/whites, two styles of ballcaps for coveralls/utilities, watchcaps, cammie hats for BDU's, and females have another hat or two depending on the uniform. And I haven't even E-7 and above; they have a completely different set of uniforms, usually. (But not always, and it varies by unit.)
And then things get trickier with nonstandard equipment. Things are sometimes not done by the book in the interests of expediency. I wore a base layer under my uniform through the winter because the gear I was issued wasn't adequate; some other folks did as well. There's no written record of this. Same with my Goretex boots, which were issued but aren't standard. Two thousand years pass and I've got a mummy wearing cammies who's not a Seabee and has Pepperskins on under his uniform, and weird boots. What conclusions do I draw from this?
Now, if documentation were lacking and you dug all this stuff up, what kinds of conclusions would you draw from it? And that's assuming you dug up everything. If you found only half of it, or a quarter, you might come to some very incorrect conclusions. Even if you found it all, you'd probably wind up making some bad guesses; they'd be the best guesses that you could make, but they'd still be wrong.
To make matters worse, the examples of equipment (specifically weapons) that survive to this day may not be the best examples. What's more likely to survive two millenia? A sword that was taken out and used, or a sword that was made for decoration and stored indoors? An old military camp is excavated and some swords are found in the trash pit that look different from any previous examples. Before we draw any conclusions, we must ask the question: "What were those swords doing in a trash pit?"
When I read about recovered artifacts, I try to keep this in mind. I'm seeing some of the picture - maybe even a lot of the picture - but I'm probably not seeing the whole picture, and even if I was, I might be coming to the wrong conclusions based on what I've seen.
We can make guesses, and we can make informed guesses, but at the end of the day, we're still just guessing.