on a partially sunny day they could have 2 5 hours with a break- in nature. The thinking behind 5+ 5 being better than 10, is that the plants max out pretty quickly under high light, can't store any more energy-think of them kind of like a battery, they have a capacity. Yes they are constantly growing and increasing that capacity, but they grow whether the lights are on or off, because they store that light in the form of carbs. so lets say you had 5 hours, then a 2 hour break then a 5 hour light, their night time is only 12 hours long, where as with a 10 hour light chunk would have a 14 hour long night, where the plants might tap out all of their energy supplies, thus slowing down growth relative to the 5+ 5. In the 5+5 as long as the plants don't "empty their reserves" during the 2 hour break, they should in theory grow as quickly as a 12 hour photoperiod. So why not just do the 12? well the algae is not as good at storing energy as the plants, so the 5+ 5 would give you fewer algae problems than the 12.
granted, as with almost everything in this hobby-this is all just speculation at this point...But anyone with hard evidence feel free to chime in.