Two cables with identical connectors at both ends can behave completely differently. The plug shape was standardised; what runs between the plugs was left to the manufacturer.
The connector is a shape, not a specification
The physical port defines pin positions and mechanical fit. It does not require that every pin be connected or that any particular capability be present.
A minimal cable connects only what is needed for basic charging and slow data, which is legitimate and considerably cheaper to make.
Because both cables fit every port, there is no physical feedback telling a user which one they are holding. The failure mode is silent: everything works, just slower than the hardware allows.
High-speed data needs extra wiring
Fast data transfer uses additional pairs of conductors that a charging cable omits entirely.
Those pairs must be shielded and length-controlled, since high-frequency signals degrade rapidly with distance and poor construction.
This is why fast cables are usually shorter and thicker, and why a long thin cable is almost never capable of the highest data rates.
Power delivery is negotiated
Devices agree on voltage and current through a conversation over dedicated pins before significant power flows.
Cables rated for higher current contain a chip that reports their capability, and without it the negotiation settles at a lower level regardless of the wire's actual thickness.
A charger and device both capable of high power will therefore charge slowly over a cable that does not declare itself, with no error to explain why.
Alternate modes carry other protocols
Display output and other protocols travel over the same connector by repurposing pins, which requires those pins to be present and correctly wired.
A cable that charges a laptop perfectly may carry no video at all, because the wires that would have carried it were never included.
This produces the familiar situation where a monitor works with one cable and not another that appears identical.
Labelling is the only reliable guide
Certification programmes and printed markings on the connector shell exist precisely because capability cannot be judged by appearance.
Keeping the cable that came with a device paired with it, and labelling the rest, avoids most of the confusion at no cost.
Where a cable's provenance is unknown, treating it as charge-only until proven otherwise is the assumption that produces fewest surprises.
Testing tools that report negotiated voltage and current exist for a few pounds, and they resolve in seconds what no amount of inspection will.