Hi Xramon,
I believe there are a few reasons for these variations:
1- What type of CLT is chosen for the project, as the carbon intensity can be ranged from 0.15 kgCO2-e/kg to 0.4kgCO2-e/kg. Please see the following report by IStrucrE. This also applies to concrete, what type of concrete mix designs are compared with CLT.
2, it is reasonable to say that CLT slab requires a higher volume of products in comparison with concrete, as a result, the kgCO2-e of the project would highly likely be equal to or higher than concrete if we normalise per volume of products (assuming higher range carbon CLT with normal of low range concrete). PT-Slab for the concrete building may be between 180 and 250 mm thick, while the CLT equivalent would most likely be over 380 mm thick. These numbers are based on high-level assumptions and they can be changed across projects.
I think it would be also good to normalise the overall carbon of projects per GFA (m2) or GBA (m2) area instead of the volume or mass of products. The main point of saying GBA is sometimes basement areas are not included in the GFA. It is good to be double-checked with your cost planner.
All the best,
Mehdi