I think I know what the problem here is.
When your defence comes to an island, they spend the first 5 minutes "looking for the treasure", as explained in the Help. That means that when you send them for 1 hour, they are actually collecting for only 55 minutes, thus getting 55 / 60 * 26 = 23.8333 (which rounds to 24). If you send them to work for 1 hour and 5 minutes, you will get 26 rubies, in 2 hours and 5 minutes it'll be 52 rubies, etc.
I tested this on my acc and it fits (my extraction rate is much higher, but the maths holds).