The number of semaphores relates to the number of options you can be routed to from that signal onwards. In this case the tallest means the route "straight" along the leftmost track of the set of four main line tracks. The smaller semaphore indicates that you are clear to proceed and will be routed to the next "option" to right, which is the third main line track from the left. The "options" are the same for the signal on the right, i.e. when coming down that track. The reason why there are only two semaphores instead of four is that half of the main line tracks are designed to be run in the opposite direction, thus you won't ever be routed to them from this signal and therefore no semaphore is required for them.
This is actually trickier to explain than I thought!