I think that's why it works -- if you change that = to a %=, the effect no longer happens.
It does make (intuitive) sense, though, because the only time that makes a significant difference is when a lot of fireflies flash around one firefly simultaneously, in which case the firefly wouldn't decide to flash twice in a row but instead interpret it as a singular flash. And if a firefly sees another one flash when it's about to, it would naturally want to synchronize its flashing with the other.
It does make (intuitive) sense, though, because the only time that makes a significant difference is when a lot of fireflies flash around one firefly simultaneously, in which case the firefly wouldn't decide to flash twice in a row but instead interpret it as a singular flash. And if a firefly sees another one flash when it's about to, it would naturally want to synchronize its flashing with the other.