That is correct. When you get lucky and mybinder.org assigns you to the GESIS cluster you receive more RAM than we promise. You should not rely on this though as one of the things the mybinder.org federation aims to do is provide uninterrupted service even if a single cluster is temporarily not available or out of resources.
If you absolutely need 8GB then you can setup your repository to launch on notebooks.gesis.org all the time. The advantage of this is that you can access more RAM, the downside is that there is only one (very reliable) cluster available to launch the repository.