It's a feature
The reason why it does that is because as you can see from your image the village is quite far (1h29 mins to be exact) this means that the merchants can't make it back in before the next shipment leaves and the game tries to hand hold you a bit by spreading out the merchants.
The feature is "ok" but it's one of those features that protects the newbies a bit from themselves but hurts experienced players for no reason.
You can go around the limitation by creating 2 identical trade routes and having one fire on even and one on odd hours. Just remember that your merchants need to make a full lap before they are available and with that distance it means if you set more than 1/3rd of your merchants on any particular mission it will run out of merchants for the later sending times.
In your case it would mean 1 merchant per hour maximum (since you have 5 available) or otherwise some trades will be smaller than others.
ahhh I get it.