Stamina mode monitors your usage behaviour and provide approximation based on that. So it makes sense if you are a heavy user for stamina mode not to rediculously inflate to e.g. 4 days, where realistically you will only last one day cycle.
Note that the mode basically will switch off your data connection etc when on standby so if you put whatsapp on exemption list, then automatically you are telling it to keep data connection on at all times. It is then expected that there will not be little to no difference between switching this on or off.
In short, i gave up switching the stamina mode on. It is similar to any other battery saver apps, and the way it helps you save battery is to switch off apps, wifi, and data connection. I dont like it because that means i will get delayed whatsapp messages....defeat the point of instant messaging.