Better on the 29th, Sunday. One seat in each of three cars on YnM 1 and one seat in each of two cars in YnM 3.

This is at 06:30 JST, one hour after seat reservation became available for that day.
Same thing at 07;15

Popular, popular, popular. Ok.
And the previous two days were weekend, Fri, Sat, and now am looking at a maybe less popular day, Sunday, and see 5 seats on the two trains.

There are dozens of unavailable seats on the two trains that day. What are the chances that all of those seats sold out in the first hour and there were five seats left over at 06:30 - and 45 minutes later those five seats were still available. The one seat on the 27th hasn't sold yet.

I guess that can be explained that there were bound to be orphan seats. I don't buy that. It is probably due to tour group bookings or JRK is holding some seats back to sell at a closer in date.

I don't really need to know how the sales happen. I paid my money and will take my chances on the day and worst case is I take the Yufu 1 or some local.