>>>> That's because people who build the stairs works with that order
>>>> of tolerance (mm).
>>>
>>> So... what the hell do you do if you build some stairs, and the sement 
>>> sinks by 0.4 mm while drying? (A perfectly likely outcome.) Then each 
>>> step might be 0.4 mm out...! No noes! o_O
>>
>> Who secures stairs in place by resting them on unset concrete? :-S
>
> I was thinking more that the *stairs* are made of concrete...

Oh you mean like one big pre-fab set of stairs?  If they all shrunk 0.4mm 
after they were taken out of the mold then the usual idea is to make the 
mold 0.4mm bigger than needed...

Anyway, who's going to notice if the last step is 0.4mm less that the 
others?

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