There's a few things in play here and not knowing the full details of the network I can only say in broad terms.
They are correct that internet plans are "up to" speeds so as long as it falls between the lowest guaranteed speed and the "up to" speed then you are getting what you paid for. They also use this to up-sale you to a higher priced plan with more speeds; it is what they are paid to do in part.
You also could have an older modem or one of their "custom" in-house/branded ones. These are usually sub-models of ones you can buy off the shelf. All ISPs have to post a list of modems that are compatible to their systems and speeds and allow consumers to BYOM(odem) for use. If you have to call to get things provisioned versus an automated system, some will try to tell you something about it not being compatible or that it can't do the speeds you have. I always have the link ready so I can direct them to their documentation. Again, it is likely innocent to get one of theirs in your hands per training. As a bonus, since they don't own it and can't lock it down as much as their own, you can usually get better average speeds and sometimes 20-50+Mbps over your "up to" limit. I honestly have no idea what they are talking about with the "points maximum" comments; they make no sense and Nest Wifi ports are 1Gbps max. If you are referring to their modem, it is likely a setting they have that could be throttling the speeds due to the router connected not being theirs.
As far as the direct connection of your PC, it could be like described above or a setting, hardware, or issue on the PC itself.
