In brief, Type-C cable specs are nearly independent from USB data and USB Power Delivery (Type-C can support anything).
By specs, Type-C connector/cable provides two methods of self-identification.
First one is by combination of resistor pulls-up and pulls-down on both sides of CC wires. Since there are two CC pins in Type-C connectors, and several analog levels, many combinations are available. Other than connect function itself and basic determination of source-sink roles and power capacity, several “alternate” functions are defined, as Display Port, Audio, and Debug mode.
The second amendment to Type-C is the mandatory embedding of e-markers into every C-C cable. >The information in e-marker contains name of manufacturer and current carrying capability of particular cable.
The state of CC1/CC2 pins is usually acquired by a special CC-controller chip. The CC-controller communicates with main system by I2C interface. The register-based interface is standardized in Intel document USB Type-C Connector System Software Interface Specification
So, many manufacturers are including markers with maximum settings that exceed the wires’/cables’ capability.
It’s not a failure to default to a safe level if it can’t negotiate properly. That’s a feature.
That’s if the manufacturer doesn’t just install a max wattage chip in every cable.
Unfortunately this is not uncommon.
I’m not extremely familiar with the USB-C handshake, but isn’t it between the device and power supply?
It used to be that simple. You could put a certain value of resistor between two pins and that was that.
This guy covered it quite well
https://superuser.com/questions/1555520/what-can-a-program-find-out-about-a-usb-c-cable-attached-to-the-computer
So, many manufacturers are including markers with maximum settings that exceed the wires’/cables’ capability.