The "unexpected" nature of the Streaham attack led to a delay in police response, Britain's most senior police officer said.
Responding to questions over how Sudesh Amman was able to stab two people while under close police surveillance, Metropolitan Police Commissioner Cressida Dick said such operations are not "man-to-man marking".
Speaking in general about the tactics used, she told the London Assembly Police and Crime Committee: "They are conducting covert surveillance, so they are not of course providing man-to-man marking. They are there covertly and that is a deliberate thing.
"It is inevitable that there could be a time delay before somebody totally unexpectedly does something."
She added: "I wish I could assure the public that everybody who poses a risk on the streets could be subject to some sort of thing that would stop them being able to stab anybody ever, but it is clearly not possible