I will be interested to read what others may write about your model.
You may find it helpful to plot each plot with a Phase of 180 degrees. Once you do that, it should be clear that the 3.6e-2 Hz mode is sloshing where the levels rise and fall at each end out-of-phase with each other (one rises while the other falls) while the center does not change. The 7.1e-2 Hz mode is where the two ends rise and fall in-phase (both rise and fall together) while the center moves in the opposite direction.
I suggest you remesh with global elements half the size, look at the results, then make the elements half the size again and check the results again.