D&D 5E The Math for the Building a Stronghold Downtime Action is... odd

[Sword of Spirit]

wow, threadcromancy indeed! I'd like to think you're correct about it being a mistake, since having things take a little less than 50% additional time seems far more reasonable. But I have to point out that errata for the DMG does exist and never changed this -- likewise the rules are the same in electronic formats like D&D beyond. I suppose in the new revised rules they'll be replaced with those (hopefully improved) bastion rules.

I wouldn't put much weight on the lack of errata. If you look at the urban encounters table it's a 2d10 roll, which means the results skew towards the middle of the table, unlike with 1d20. But the table results are just in alphabetical order, indicating it was clearly not arranged to be weighted.
I even reported this way back when.