Think of the lens as a gear. If you shrink one side (zoom in), the aperture is forced to get smaller. You’re most likely rocking a 28-135mm lens, which is a manual lens that forces the f-stop change, no matter what, when you zoom. If you’re at 13, locked down and pulled out, then slowly zoom, the internal gears will force the fstop to go to 14 and then likely 15 when you’re zoomed in full.
I was mystified too – thinking of it like a gear helped me understand it. More expensive lenses have better gears and don’t require this. Although, I rarely zoom – it’s more about sliders and cranes these days.