Miyagi (Rich Anti-socal loner girl) has complicated feelings toward our popular girl (MC).
I guess to Miyagi, Sendai (MC) is pretty much the closet person to a friend/best she actually has so far I am assuming this...Miyagi you don't hate Sendai...You just have messed up confused feelings (Curiosity, jealously and satisfaction, frustration, surprise, yearning and longing for emotional connection, amusement + minor of other emotions probably mix in) that your not able to accurately describe and judge rationally due to being too young and inexperience in life lol XD
Sendai has great eye for attention for noticing the minor hair cut...As expected of one of the popular girls in her school...You need a keen eye and able to judge the room to be polite/make the correct social actions to not be hated/out-cast/seen as weird. Too much work to be popular and needing to have some extrovert energy~
Like the comment above me said (Genuine emotion connection), humans can be influence and read minor micro expressions on other humans' faces...It not perfect and their brain can falsely read the small facial/micro expression but it is kind of accurate 50% of the time.
Miyagi probably feels a tiny bit of superiority at seeing sides of Sendai that most people probably won't see (Disgust and annoyance instead of polite calm smile-Public face/ Different masks and persona depending on the location and people involved.) You can see and feel her minor shock/surprise at how Sendai is offering to cook/make her food without any orders/ulterior motive instead of going along with their current 'game type' relationship due to 'money'...That is usually something done with friends or people you enjoy being around.
Otakus with that weird niche fetish seeing Miyagi asking MC/Sendai to look at her with a disgusted look: "Could this girl be one of our people?"