I'm a complete layman here, but here's my guesses.
The first problem is that generally when talking about weather predictions you're talking about a large area. Asking, "will it rain in New York?" can be a loaded question if you're talking about a scattered storm. My understanding is that a lot of the time when you hear 60% chance of precipitation, the weatherman doesn't mean 40% chance it won't rain in the area, but that roughly 60% of the area or population will see rain. So the first advantage this app has is that it's forecasting for you individually in a very specific location, rather than for an entire TV or newspaper market.
Secondly, I think the short time frame is actually much easier to predict. On a short timescale you basically just look at the radar for the past 15 minutes, and see where a storm if any is heading. I do this all the time with weather.com. Open up the radar, see if a storm is nearby and where it's headed. It seems much harder to predict what a storm will do hours or days out, than what it'll do in 15 minutes.
The first problem is that generally when talking about weather predictions you're talking about a large area. Asking, "will it rain in New York?" can be a loaded question if you're talking about a scattered storm. My understanding is that a lot of the time when you hear 60% chance of precipitation, the weatherman doesn't mean 40% chance it won't rain in the area, but that roughly 60% of the area or population will see rain. So the first advantage this app has is that it's forecasting for you individually in a very specific location, rather than for an entire TV or newspaper market.
Secondly, I think the short time frame is actually much easier to predict. On a short timescale you basically just look at the radar for the past 15 minutes, and see where a storm if any is heading. I do this all the time with weather.com. Open up the radar, see if a storm is nearby and where it's headed. It seems much harder to predict what a storm will do hours or days out, than what it'll do in 15 minutes.