The gif frames are "left, off, right, right". I.e. there are only two flashes, but the second lasts twice as long. I assume somebody either didn't understand how gifs work, or they accidentally dropped frames in export.
Note that there is also a version of this that has two flashes and the illusion is a third flash. The gif, as described, is intended to be the version where there are three real flashes.
Note that there is also a version of this that has two flashes and the illusion is a third flash. The gif, as described, is intended to be the version where there are three real flashes.