Why are different screw heads used in UK 3 pin plugs?

This morning I had to take my fan apart to fix it and while doing that I noticed the rubber bung on the three pin plug that gives you some resistance from pulling the wires out of the plug had come loose.

So, I went to unscrew the plug so I could pop the bung back in but then noticed that there are different types of screwheads used in the plug?

The main one to unscrew the back of the plug from the main body needs a flathead screwdriver but the one that you need to use to unscrew the bar across the cables is a Phillips head?

Why is that? Why are they different? Is that done deliberately?