Air temperature is not the issue, the issue is the comparison of the water temperature with the available solar heat at the solar panel location. This means that solar heat being available depends on a number of factors, not only if the sun is out, but also how much heat at the panels may be getting lost to wind cooling, etc.
Picture a situation like this, a calm spring afternoon where the water temperature is 70 degrees, the outdoor air temperature is in the low 60's and the sun is shining, just like the roof a black car on a day like this the panels will be much warmer than ambient, and also warmer than the water. The solar controller would sense that panels being at least X (usually 2-3) degrees warmer than the water and divert flow through them and therefore warming the pool.
Now picture that the wind starts blowing at 10-15 mph, this cool air crossing the panels cools them down equal to or below the water temperature, which lets say is 71 at this point, the solar controller turns the flow to the panels off and waits for the wind to stop and the panels to warm up again. Repeat.... (the same thing would happen with clouds, afternoon rain showers, etc.)
Ike
p.s. the better solar controller have a built in delay feature to keep short cycling of the pump motor from occurring.
Also remember that you can get a basic GL-235 without actuators for somewhere around $150-$175 online with a bit of shopping, this includes the housing, and the controller, and complete kits with valve, an actuator, and a pair of sensors for about $250 (search: GLC-1P or GLC-2P solar) for the kit