It is not unusual for this situation to arise if you do have a heavier tap, as the weight of the tap can cause a stainless steel sink to 'flex'.
This is not caused by the tap or the sink being defective in any way and can be rectified by your tap being braced. You are able to purchase a tap brace or make one quite easily.
Franke have recently produced a new patented tap brace which fits directly to the worktop and has a variable collar to fit all models of taps.
To make a tap brace you will need a piece of wood or chipboard, which can be screwed across the underside of the worktop cut out. The tap can then be fixed through a hole in this so that the sink is supported by the wood, preventing the flex due to weight of the tap. With a tap brace, the tap is effectively secured to the underside of the strut and not to the underside of the sink to reduce the tendency for the sink to flex, particularly when the tap is turned on or off.