It’s from the early days of rolling out CCS plugs alongside the European/UK Model 3 launch in 2019 at Superchargers. They went for a breadth vs depth approach initially by adding 1 or 2 CCS plugs per location before going back and doing the rest. These signs were added so S & X owners didn’t take the only CCS a plugs.
Didn't half annoy some people at the time. at busy locations there was the odd Model 3 owner turning up and telling Model S or Model X owners they should move even though there were no free charge points for them which wasn't the intent.