That is too much difference for it to be just calibration that is needed. Something else is wrong.It is actually lower than what we expect isnt it? 10kg is 10000 gram, right? So giving me 6000-7000 is actually giving me a lower value then i expected. Could this be a energy value though? Maybe i need to scale this so that i can find my actual weight.
I suspect that you need to set the transmitter to match the mV/V of the loadcell. On the side of the transmitter it says range 1-10mV/V, so it must be configurable.
If for example, the loadcell has an output of 3mV/V and the transmitter is set to 4mV/V, then it can explain the greater part of the error you are seeing.