In the main activity, you measured the I/O pin high voltage under load.
In the Try This section, you measured the I/O pin high voltage with no load.
In both activities the I/O pin low voltage was not under load because it was trying to supply 0 V to a circuit that had 0 V (GND) at the other end. No electrical pressure was applied, so no current was flowing through the circuit. Thus, no current “load” on the I/O pin.
What I/O pin voltage under load did you observe? What is your explanation?