How do I decide the resistance of a DC solenoid valve?
You don't wish a series resistor. You want a supply that can provide 24 VDC @ 0.1 A. You can think of the device as having resistance in the 206-280 Ohm range as you wish. But that's what it is, not what it requires. See PeterJ's comment.

I might guess the difference in your calculations is rounding. Or there may be a maximum power the part can take, to allow for voltages a little higher than 24 VDC, but it doesn't need that much to operate.