To describe the battery correctly, you introduce a battery, which consists of two parts...
In fact, you are now able to properly explain your measurement results, as you can see from the circuit diagrams shown.
The internal resistance is not only a theoretical concept. Repeat the experiment 4 and leave the circuit connected for about about 10 minutes. The battery itself heats slightly up. This heat is generated by the internal resistance.
In the next experiments, you examine the consequences of the internal resistance.