Wizards seem to be overly skeptical of the information that they get from their magical detection spells, an analogue to what some muggle scientists call algorithm aversion. (As evidenced by the current confusion about Hermione's nature, and the lack of response when the wards previously identified The Defense Professor as her killer.)

This means that scheming wizards who want their plots to go undiscovered don't need to trick the magical detection spells, they just need to pursue strange enough plots so that other wizards won't believe what the detection spells tell them. Which makes Voldemort's creative uses of magic analogous to Dumbledore's ploy of pretending to be crazy.