You are forgetting that the containers which carry the cargo also weight something. There is your difference.
You might be sacrificing some payload for range look at the performance tool to see how much payload you actually have on the route. It is also the weight of the containers like Rubio said.
And the total cargo capacity stated in the figures is only a direct translation of the volume available inside the aircraft. This does not necessarily mean you can completely stuff the plane full with standard containers and the full volume could only be used if you transport cargo with a lower density and weight.
For a more pronounced real world example, the Airbus Beluga has a cargo volume capacity of 1,400 m³. In the AS formula this would translate to roughly 217,000 kg of cargo capacity. The maximum payload is only 47,000 kg, which is less than a quarter of what would theoreticaly fit, but enough to transport fuselages which are large in volume but low in weight.
There is nothing in cargo area but experience....
Because cargo is not for the faint of heart…
I asked quite similar question few weeks ago, right after Quirky restart. Seems like AS has new calculation formula for cargo, it’s just not documented. From my current experience “tare rate” is the same for all types, but I’m yet to find some courage to try if “container only” setting will further decrease capacity.
It would be nice if game has a bit more explanation and tools for cargo. Currently I’m eagerly waiting whether new bigger type will break my schedule due to larger turnaround times, which are unknown before you actually schedule the plane.