Sorry I don't have the time to dig into this more deeply right now. I'm
sure Owen has done a great job in estimating loss, but here are some
things to think about:

1. Belden 9204, like a lot of other 75 ohm cables, has a copper-plated
steel center conductor for strength. At 3.8 MHz, depending on the copper
thickness, current might be entering the steel. If it is, the loss will
be a lot more than a simple model for solid copper would predict. I
notice that the statement at the bottom of the data you posted says
"Loss model source data frequency range 10.000 - 1000.000 MHz". You're
well below that. A good reason for a lower limit on the model would be
not accounting for current penetrating into the steel.

2. Some common RG-59 type cables have stranded center conductors and tin
plating. Both increase the loss. More importantly, stranding results in
much thinner copper for a given percentage of wire diameter of copper
cladding.

3. A logical way for a cable manufacturer to cut costs is to put a
thinner copper cladding on the center conductor. This would have no
effect on the performance at VHF and above, where the cable is most
likely to be used. So thin copper wouldn't surprise me.

The only way to really know the loss is to measure it. And this might
not be the reason for any apparent error. But it might be. As Tom said,
though, 10, or even 15% deviation from nominal isn't unusual.

Let me relate a story. Years ago, I came across a very large surplus
quantity of approximately 0.1" diameter 75 ohm cable. It was just before
Field Day, and because it looked to be in good physical condition, I
measured off 100 feet, put a couple of BNC connectors on it, and tossed
it into the pack as feedline for the 40 meter antenna. (I backpack my
gear on Field Day, so weight is a major consideration.) Field Day went
ok, but it was one of those years when we were just at the other
stations' noise level, requiring a lot of repeats, QRZs, etc. Afterward,
my FD partner was saying that all we needed was another 2 or 3 dB gain
on 40, and we'd do a lot better. I agreed. Not too long afterward, I was
measuring the impedance of a folded dipole through a half or full wave
of that 75 ohm coax (since I had a lot of it), and was getting bizarre
results. And that's when I first learned of the importance of cable loss
on impedance transformation. I had been assuming lossless cable for my
calculations of load Z given input Z, but got suspicious that loss might
play a role. When I modified by equations to account for loss, I was
surprised at how much difference even a little loss made. (As it turns
out, loss makes more difference when the load Z is far from the cable
Z0, as it was in this case, than when they're about the same.) I put
more and more loss into the formula until I got about what I expected
for load Z, given the input Z I was measuring. 4 dB at 7 MHz! A quick
measurement with the wattmeter confirmed that the cable did indeed have
that much loss. The problem was the thinness of the copper cladding on
the very small steel center conductor strands. Even though the cladding
was a substantial portion of the wire diameter, it was still very thin
because of the tiny wire diameter. At our next sked, I told my FD
partner that I'd figured out a way to get a couple more dB out of our 40
meter antenna. . .

Roy Lewallen, W7EL