Then they're lying to you. Unless you're on a completely separate power grid, your energy is, in reality, provided by the power source closest to you. It might be solar or wind sometimes, but the rest of the time it's probably a non-renewable source.

German companies claiming to get their energy through renewable sources are primarily buying credits from Norway and a few other sources in Europe.

You can stick your head in the ground and pretend all of the electricity you use is being generated by renewable resources, but as long as plants in Germany are producing power using coal and natural gas, it means you're using that energy.