Inside the Reactor Planner, the "Produced: # EU/t" value in the Active Simulation Info tab is incorrect, presumably because it doesn't factor in the fact that the EU output is per Minecraft tick, but in the Reactor Planner it simulates the production of EU per "Reactor tick", which only occurs once a second, instead of every 1/20th of a second (Minecraft tick).
This leads to the misconception that a single Uranium Rod only produces 200,000 EU before it is finished, rather than the expected 4 Million EU.

In the Setup Result Prediction tab, it has the correct Total Power Created value of 4,000,000 EU.

This obviously doesn't happen with Steam Reactor simulations, as the Steam Reactor doesn't produce EU.