The unit of trade is not the ton but the cannister. So you have 1 cannister of crop harvesters and 1 cannister of slaves etc.
Admittedly for some of the items that sound as if they could be quite large it makes you think you couldn't even fit one of them in there but that's what we have. Perhaps a crop harvester in the future is not like the hulking things we have now.
One canister does weigh exactly one tonne. My guess is that it simplifies the hyperspace route plotting, or something like that (if you don't take the exact mass in account you risk getting misjumps or something).
And for crop harvesters... maybe all machinery in 3300 is IKEA-style
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