Hermione expected a house-elf to answer the front door after she rang the bell. None of the Lestranges had ever been much for completing even simple tasks for themselves. Gemma Farley assimilated to her new lifestyle quite quickly after her arranged marriage to Rabastan was finalized. So when Madam Lestrange was the one to open the front door, Hermione was startled enough that it took her an extra second longer than it should have to disarm the frightened and confused witch.