If you ask a Sim to sit in a specific chair then they should do so.

If you ask a Sim to perform an action that does not demand the use of a specific seat, such as reading or eating, the Sim will choose to sit in the chair closest to where the action is to be performed.

Sims will decide to use a surface that is right for the action that they are performing (bills will go on some sort of desk first, then a table or end table).

If several surfaces are available, they will choose the closest one that does not conflict with the path of another Sim.

If they continue to perform actions in the same odd locations, break their rut by rearranging the furniture.

If you want more control of your Sim, turn off the “free will” option located in the “Game Options” menu.