The Octopus Tree's frightful presence says (p67):
All creatures that have fewer than 26 HD must make a DC 26 Will save each round they remain within 30 feet of the octopus tree or become shaken as long as they remain in this area. A successful save leaves that opponent immune to that octopus tree's frightful presence for 1 day.
Aren't those two sentences mutually exclusive?
Not exactly. For example, you could fail your save, become shaken, leave the area (becoming "un-shaken"), then come back (forcing another save vs. frightful presence). A successful save doesn't end the shaken effect, but it immunizes you for the next time you enter the area.