Choosing a scenario also sets the OoB and weather for that battle from the scenario file. The weather might be different from the weather set in the map.
Using a scenario is a very quick way to get the OoB for the battle, and in at least one case the OoB isn't the same as the map name.
I'm not sure why Berezina isn't in the scenario file, but there's no OoB with that name, so maybe it's missing at the moment.
Edit: Wagram in the scenario file has weather set to Summer hot. The map has weather set to Summer rain.
Hook