Most large corporate buildings have a DAS system, which is a distributed cell system in the building. This makes cell phone reception excellent inside the building. This allows the cellphone to set its cellular transmit power to a very low setting, saving power and extending battery life. I'll bet cell phone testing occurred in this type of environment.
If you are in a brick building, or a basement, and the building does not have a DAS system, then the cellphone needs to set a high cellular transmit power, reducing battery life. Being associated to a weak WiFi signal also causes the WiFi antenna power to be increased, reducing battery life. It can be amazing watching the battery percentage drop quickly in some of these environments.
Having no connection can be superior to a weak connection from a battery perspective. In that case the phone will periodically probe for connectivity but use less battery than a weak cell or WiFi connection.
Airplane mode can be helpful in these situations. You won't be able to use your cell connectivity, but your cellphone battery will not drain quickly. You just need to remember to turn off airplane mode when leaving the environment.
Which situation is "correct" when calculating battery life? There is no _correct_ answer. Your personal battery life will vary dramatically based on the cell and WiFi strength in your location. This is outside the control of Apple. Apple does it's best to optimize battery life by modifying transmitter power for the situation.