I noticed the same apparent error, and decided that perhaps Ismay and Bea had tried to recreate the incident in the girls' bathroom, since Ismay had earlier recorded that there were two in the house. But it would have been nice to have had this made clear. I noticed quite a few other errors too. For example you can't order goods over the telephone using a pin number. The retailers would want the entire sixteen digit code. Also at one point Marion changed the locks (page 48 in my edition), but Fowler was still able to get in with the old keys (page 71).
I disagree that this is not Ms Rendell's fault, she is the writer after all. But the proofreaders are not doing their jobs properly. Errors in her books frequently spoil my pleasure in her tales, although I found the errors less irksome here than in 'Tigerlily's Orchids' for example.