Download numbers are always funny without creepy telemetry: without unique IPs, and weeding out CI machines, even the relative numbers can be skewed, but 80/20 is not surprising.
Another wrinkle: every install of jupyterlab also incurs a download of notebook, even if it’s not being actively used, bringing it closer to the old rule-of-thumb of 80/20.
Further, the (troublesome) jupyter metapackage triggers a download of notebook and not of jupyterlab… but does bring in qt 
, which is kind of a branding fail.
After Notebook 7 is released, the dependency will be reversed (as it will be built on lab), so these relationships will get even murkier.