There are several mistakes
There are several mistakes here. Notably, the 2010 Prius only had 35 miles of range, not 38. Plus, the picture shown is of a gen-2 Volt which has 53 miles of range.
The most likely reason the Prius Prime has 25 miles boils down to cost. They wanted to use the same modules that the regular Prius Eco had, only the Prime has 5 modules instead of 2. They could have technically fit more cells and better used the space in the back of the Prime, but it would have cost more money to engineer it and to manufacture it. Not only because of the extra cells, but because the shape of the pack would no longer be a simple box, but rather an oddball shape.
The big problem with the Volt was cost. It was never profitable for GM. But Toyota has designed the Prime to be able to sell at a profit. That's very important.
But that begs the question, why aren't they selling more of the Prime than they are? There is plenty of demand and many parts of the USA have no stock available. This is where I give credit and agreement with the article that Toyota is holding back because they are wanting to validate the technology. Prius Prime drivers are essentially beta-testers for Toyota to see how the batteries hold up being charged and discharged every day. If Toyota is satisfied with the result, I expect to see more range in future versions as the design is refined. The Rav-4 Prime is a good example as it will have 39 miles of range.