These are equivalency measurements to help understand the order of magnitude of a blast. In terms of margin of error, “a few hundred tonnes” vs 1.29 kilotonnes is close enough.
The actual strength of a blast will depend on if it got hot enough to have secondary chemical reaction, the shape of the container it is in, the height above ground (which is why nuclear bombs are detonated at altitude), topographic effects, air density etc etc. Experts and grad students will right now be amusing themselves running models on the blast and the many external factors, and each will come up with an answer in a range maybe plus or minus 30% of each other.
Equivalency is to help us visualise or compare, and is not an actual physical property of the event.
It’s like me saying “this bag I’m carrying is about as heavy as a car battery”. It helps you understand the weight, but is not itself a fundamental property or unit of measurement.
Useful paper on blasts in urban areas, that discusses limitations to various estimation/modelling methods in first parts. https://ro.uow.edu.au/cgi/viewcontent.cgi?referer=https://www.google.co.nz/&httpsredir=1&article=1360&context=engpapers&referer=www.clickfind.com.au