Why SQL Server Standard 2017 is using more memory than limit described in documentation?
I believe you missed the documentation. Please see the highlighted points in image below



The memory used by memory optimized tables is 32 GB PER DATABASE

So let us say you are having SQL Server standard edition having columnstore indexes and having memory optimized tables in 3 databases. 

The maximum memory utilized can be

128+32+(3*32)= 256 GB. 

Let me know for how many databases you are using memory optimized tables ?. The 128 GB limit is only for buffer pool