The Section X Conundrum: A Paradox in In-Laws

I'm having a hard time wrapping my head around Section 125 of the Code of Criminal Procedure 1973 (CrPC) which deals with maintenance for wives, children, and other dependents. Here's the part that's got me puzzled: while it states that a magistrate has discretion to order maintenance based on the 'reasonableness' of the amount, it goes on to specify an amount equivalent to 1/3rd of the husband's net income as a 'presumed' amount. Is it just me or is this essentially contradictory? If 'reasonableness' is the yardstick, how can we set a fixed ratio like 1/3rd? It seems to negate the very concept of discretion.