I think it's because Amanda says in her e-mail that before leaving at around 4 p.m. Meredith either took some clothes out of the washing machine or put some in, she couldn't remember which. So since the clothes in the machine were Meredith's, it's assumed she put them in shortly before leaving for her friend's.
Of course, that could all have been a cunning plan by Captain Amanda to cloud the time of death issue by pretending Meredith put the clothes in earlier. Lucky for her Meredith hadn't used her computer or made any phone calls/texts after coming home either, so the washing machine lie went undiscovered.