Ask UK

I'm really just guessing but I read that there's a bit more to it when installing these lights. Oftentimes, engineers set up a timing system so that a series of lights will turn green (to cars) in a chain, allowing traffic to get through and not idle a lot on a long, straight stretch of road. So first it's green on one end, then assuming cruising speed the second one turns green a bit later a few hundred metres down, then the third a bit later, and so on. I'm assuming buttonless, automatic lights.

Now, I can imagine that these work the same way, only that they don't turn green (to the pedestrian) when nobody is around, pressing buttons (making the traffic pass even faster, reducing the amount of fumes they leave). Once you press the button, maybe it still tries to maintain the same timing and it waits for its turn.