I'm no plant expert...
...but I've stayed at a Holiday Inn Express.
That being said, I'm guessing(totally no scientific basis to back this up) that when the water levels pick up and flow increases in P1, it'll be able to move onto P2 and P3. Right now, there's nowhere for the duckweed to go.
It's on P1 and P3 now, skipping P2. Ah, the mysteries of life. It makes for some really cool photos, though!
I read that it is consumed in some countries. Duckweed salad, anyone?