The thunderstorm layer is expressed in lightning per square kilometre. It is the average density of likely lightning strikes computed on a wide area but then expressed per square km. So it is not within a radius to your location, just a potential density.
Finally I understand that ECMWF compute the potential of lightning strikes density during 24hours but they are displayed at a precise hourly time frame. It is about the same when you are driving, your speed is expressed in km per hour (or miles per hour), but your speed may vary by each minutes.
Weather forceast model is a fascinating topic ! The more you use them the better you understand them, you know when one model can be more accurate than another, what model can or cannot do. IFS is probably one of the best model to predict the changes at synoptic level (1000 or 2000km) when Arome is capable to solve and calculates convection which can be useful to predict thunderstorms.
Of course we can keep using one model only but Windy offers the possibility to easily compare the results from the different models so it is a nice and easy way to check the consistency between the models and therefore give you more confidence about the weather forecasts.