It's not that difficult to get an accurate measurement for a model if there is an actual full size aircraft still in existence. Many model companies will measure and photograph a real aircraft as well as use actual builders plans. But this data isn't always available so it is approximated from the varied sources, hence the discrepancies. Newer models such as the Airfix kits are created by first scanning the aircraft and creating a computer model that is accurate to less that a millimeter. I'm sure Wingnuts kits are developed using the same plans that TVAL uses to build their reproductions.
George
Do you have a 3D scanner to scan real aircraft?
And if you do have, do you have the historic aircraft at hand?
And if you have them, do they let you measure them?
The "it is not that difficult" is rather questionable.
You can measure aircraft with the right instruments, but probably you won't reach the needed accuracy.
Photos can help here.
I measured a bleriot, and a spad VII some thirty years ago and a gabardini replica three years ago, but I was lucky to find fellows modellers mounting the guard there.
Last week I went to the Italian Aviation museum, but the climate was so hot that I had to go out in half a hour before my brain evaporates...
You will find any sort of difficulties in measuring aircraft...
Moreover, many designers work from their home, and so it is mostly a research work not an empirical work...