It's more likely that they haven't changed the formulation. For the most liquids, as the temperature increases the viscosity decreases. This means that there may be the same amount of oil in the same place, but it doesn't stay there for long. The less viscous the oil, the more mobile it becomes, so it might be that during the colder weather, the pattern holds up longer making them seem wetter, meaning you can use the solid ball. In the hotter weather, the oil moves faster giving the appearance of a drier lane, making the pearl a better choice.
Just my thoughts....
--------------------
Reporting from England