A big reason for that is how we verify precipitation. In forecasting, rain only counts if you can measure it to the closest hundredth of an inch. Sometimes you'll feel some raindrops, but it won't be enough to measure 0.01 inches. In that case, it is considered a trace (T), and it is counted the same as if it didn't rain.
For Saturday, my initial forecast may have been the most accurate one. Through the week, the chance of rain was looking higher with each day, since there would be a front lying east to west across central Ohio, with a low pressure center tracking east along the front. There were numerous showers and thunderstorms around the area, but luckily for those at the track (and unluckily for us meteorologists), they didn't directly hit Mid-Ohio. I was watching storms on radar most of the day, and at least twice, the storms completely fell apart within 10 miles of the track. It was like there was a force field around Mid-Ohio, deflecting all the rain.
I had been advertising a dry day on Sunday, and that worked out. There were some isolated showers north and east of the track on Sunday, coming within 20 miles or so of the track.
To summarize with gifs:
Rain forecast on Friday - nailed it: