Answer:
Possibly due to density differences between the solutions. The effect may be may also be accentuated by the presence of differing levels of nonpolar solvents.
Step-by-step explanation:
There is no description given of the different solutions. There must be a density difference among them, but it is impossible to determine that with certainty without more information. The lower density solutions will migrate upward (red in this case). Intermediate density solutions will be in the middle (yellow and green), and the highest density moves to the bottom (blue).