A question about running speed.

As you may know from my ritual of photographing the sunrise and posting these relentlessly repetitious photographs on this blog...

 

... I go out for a short run — it's 1.5 miles — every morning (that is, unless it's very cold, rainy, or dangerously windy). I've been doing this since September 2019, when I got the idea for this ritual, and I'm pleased with having kept it up. It's healthy and wholesome (and artistic and spiritual). I didn't think I was even able to run. I'd never been a runner, and I didn't even know how to breathe. I watched videos on how to breathe while running. I had to research what to wear. 

But here's my question now. I'm not concerned with running a longer distance, but I would like to go faster. My iPhone records my speed. It shows a range of speed for any given run, and that varies, but it often goes as high as 4.9 mph. And yet it never hits 5. It's like the phone is toying with me. If I can do 4.9 on any given ordinary day, why do I never hit 5?