When Hermione heard Ginny's voice she was reminded of just how long she'd been seated at that window. Just after they'd all processed the information Molly had headed to the kitchen to make tea and Ginny had remained upstairs. After taking the cup of tea, likely around seven that morning, Hermione had retreated to the spot in which she was still seated.
When Percy took the cup of tea Hermione stood. She hadn't planned on moving just yet, but flat tone in Ginny's voice spoke more than any words she could have said. It was the same tone Hermione's voice had taken. "She'll have a cup of tea," Hermione spoke not really caring if Ginny wanted tea. She simply wanted to give Percy a reason to leave so she and Ginny could share a moment of grief alone.