I have a question about how should scientific unit be written. The author uses the style inconsistently, but since they have science background, I think this inconsistency is actually a typography choice. If it's indeed a question about typography, then I think it's on-topic here.
What do you think? The proposed question is on below
In the book What If?, the author mostly write the units in full words like kilometer, atmosphere, megawatt instead of km, atm, MW. However he does use the symbols in the drawings:
In one line both mm and millimeter are even used together:
This makes me confused because I can't find a reason to not using the SI symbols. I guess the reason is similar to writing numbers in words when doing so doesn't look silly for better reading flow feeling. But in the above example, I can't explain why the first number should use mm while the second one should use millimeter.
What could be the reason for this inconsistency?