Not sure I fully follow that but I'm trying to :-)
I'll try to explain better... A bit long, though.
If you look at the "Quiet Line", "SNR per tone" and "bits per tone" graphs, you can see that U2 has relatively few bits, seemingly because it is noisier - especially in the upper part of U2.
First, check the "bits per tone" graph for your line, and for "happyhorse". You'll note that your U2 spectrum is carrying fewer bits, and none in the higher region. Around 3 bits per tone, perhaps.
In DSL, the reason that the modem synchronises with fewer bits on any one frequency is because it has a lower SNR. That means the spectrum is receiving lower signal levels or higher noise levels; the lower number of bits is the modem's way to cope with that situation. Comparing the two "SNR per tone" graphs confirms this.
Are you getting more noise from other lines? Yes. This can be seen in the "quiet line" graph (although that only measures the noise at the time of the last sync; it isn't "live").
In U2, "happy horse" is seeing noise from other subscribers of between -122 and -124dB; your line is between -116dB and -120dB - which is perhaps 4-6dB noisier; for comparison, -140dB is very, very quiet.
In DSL, there needs to be an extra 3dB of SNR in order for a tone to carry one extra bit. Your extra 4-6dB of noise means (roughly) that each tone in U2 can carry 1-2 bits less than "happyhorse"'s.
Onto the "power" graph... Unfortunately, this only shows the aggregate transmission power, rather than the power used on each tone. So we have to guess a little.
Your aggregate upstream power is running at 1.5dB lower than "happyhorse"'s. If this means the power of individual tones is lower by that amount, then your line also has a slightly lower signal level too. A lower signal that could be worth another bit here and there.
In total, we can see reasons why your U2 tones can be carrying 2-3 bits less. When aggregated over time, that could be worth 10Mbps on your sync speed.
The question is ... is any of this speed recoverable through a fault. The answer is that it is unlikely.
a) Crosstalk noise varies, and depends on the number of your neighbours that have subscribed.
Complaining to your ISP, or calling out an Openreach engineer, isn't going to get rid of those subscribers.
b) Upstream power varies too; it is forced to reduce by the DSLAM to allow distant subscribers to get reasonable upstream speeds.
Again, complaining to your ISP, or calling out an Openreach engineer, isn't going to get rid of those subscribers.
That is, I should say, all just my opinion. There could be some form of fault on your line that happens to be augmenting the noise or suppressing the signal ... which an engineer visit could fix. But there's little that stands out as obvious.
Onto the
positive news...
Going by the attenuation values, "hungryhorse" has a shorter line, and ought to get higher speeds anyway. Yet the downstream speed is actually comparable.
Go back to that "quiet line" graph, and look at the noise in the downstream spectrum. You'll see that your line is quieter by 3-4dB. That helps your line carry more downstream speed, relatively.
DSL is a matter of swings and roundabouts. You can see how much your speed depends on the noise/interference you are receiving from other lines. And that interference behaves differently for upstream and downstream.
So now time to raise with my ISP
Can anyone help me out how here explain how I frame the problem to the ISP will "missing tone in upper U2 make sense to them ?"
"missing tones in upper U2" is, in the end, a consequence of DSL trying to make best use of your line's capabilities, in your line's environment, while also trying to make best use of other lines. It is "designed-for", not a fault.
ISPs won't care about details like this. In the end, all they care about is whether the total throughput speed is within spec.