When you use the Data-defined Size Legend you can define Manual size classes for the legend.
The Value field is the size of the symbology, and the Label field is a value label that you assign to that size.
Since you are defining an expression in the data defined size of the symbol, the size of the symbol is not the value of a field, and you need to manually evaluate the expression to know the size of the symbols that you want to include as classes.
In your example, when the value of the field 'nb_com' is 10, the exrpression returns 4.65 to the size of the symbol. So you need to write 4.65 in the 'Value' field and 10 in the 'Label' field. The same calculation applies to Value' 17.26 for the 'Label' 100, and the 'Value' 60 for the 'Label' 890.
This is a manual definition of the classes for the legend. The assistant do it automatically and you have not control about the classes defined by it.