[Solved. For anyone who is also confused, see the comments under Poutnik's answer. As Poutnik reminds me, remember there is no air involved. I successfully picture that with the piston experiment in Karl's answer.]
When I read the phase diagram, I think it suggests that "at certain temperature and pressure, the phase of water would be (solid, liquid, gas) or two/three of them coexisting." But when I look up NTP on the diagram, it says "liquid", while the saturation pressure of water at 25°C is 0.03atm. So the situation can't exist since the vapor is going to condense until the pressure drops to 0.03atm.
Then what actually is the phase diagram suggesting by "liquid"? What am I missing here?
Edited: Poutnik reminds me that the pressure is the total pressure, not the vapor pressure. Then my question should be "If the vapor and liquid coexist not only at the curve, then what is the meaning of 'liquid' on the diagram?"