RE: Why variables are used on statistics pages and not fields?
Could you please give an example of what you mean for "dynamics values"?
If you mean flowfields, these are automatically calculated when the field is bounded to the page and it opens.
You could also do some exercises related to Autocalcfields property and SETAUTOCALCFIELDS function
docs.microsoft.com/.../setautocalcfields-function--record-
to better understand when and how these are calculated.
Be aware of the performance payload in always calculating fields, when you have a lot of records in the underlying tables.
Basically, flowfields are declared within a table and are part of the table / tableextension that you are using.
Variables, calculates values on the fly and might fit best when
a. these are used spot (e.g. just in one page or one report)
b. values comes from etherogenous sources (e.g. multiple different tables)
Up to you, as developer, to find out the best solution that fits your specific needs.