Hi oskar,
a) P and Q tables are not identical both have different fields as one has time dependent and another has time independent attributes.
b) in attributes tab in master data infoobject u select if attribute is time dependent or not which will keep those object in those tables. also while reading the values it goes to those tables based on flag marked for time dependency of attribute.
i guess this is clarified now. May be you can see more information in tables and change some test objects and see how tables bahave.
Regards,
Ganesh