Maybe sumie really thought that this was a transactional thing. Maybe her feelings aren't the same.
I think it's more that she's afraid of being hurt and was using the whole seven days thing as a way to protect herself. The way Manatsu asked her question keeps the temporary, trial girlfriend, framing (even if most people would get that she wants to keep going), so Sumie answered in a way that's safe for her. I suspect that if she'd asked in terms of wanting to be her real girlfriend, not as a trial and without a time limit, Sumie would have said yes. Similarly, I suspect the climax is going to have them end the trial relationship and then, shortly after that, Manatsu will screw her courage to the sticking place and give Sumie a proper confession, as if she was asking for the first time, and Sumie will say yes.
I'm also starting to wonder if Sumie's supposed to be on the autism spectrum, given her reactions to certain stimuli, her relatively flat affect, and her potential issues with missing implications or underlying meanings.