I don't remember why the two modules for xlsx files and not just one.
Last time I checked OpenPyXl doesn't deal correctly with .xlsm files - there's a parameter for that but I believe it's still experimental. In my case this meant that, on a Mac, Excel would complain that a file generated with OpenPyXl was corrupt and then successfully "recover" every generated file.
My wild guess is: your customer reads the files with OpenPyXl, processes the data with Pandas, and then uses XlsxWriter as the custom Excel writing engine.
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Last time I checked OpenPyXl doesn't deal correctly with .xlsm files - there's a parameter for that but I believe it's still experimental. In my case this meant that, on a Mac, Excel would complain that a file generated with OpenPyXl was corrupt and then successfully "recover" every generated file.
My wild guess is: your customer reads the files with OpenPyXl, processes the data with Pandas, and then uses XlsxWriter as the custom Excel writing engine.