Purely conjecture, but....
I've heard that plants are most photosynthetically active during the initial part of the photoperiod, gradually sloping off as the "day" goes on. During this inititial period, plants would probably easily out compete algae, but algae would be able to out-compete the plants later in the day when plants are less active.
I would imagine that this slow decline in activity is a result of some sort of a feedback loop that prevents the plant from making more sugar than the plant can reasonably process, thereby wasting the plants resources, both energetic and otherwise. Thus, after the plant has built up excess sugars during the earlier parts of the day, it slows down. Hypothetically, if you put a four hour gap between smaller photoperiods, the plants would have had a chance to process its excess sugars, thereby eliminating the feedback inhibition. Thus, the plant would enter the second photoperiod at full tilt again.