I am doing a Transformer experiment and I don't understand how the max voltage of the secondary is always the same while the max voltage of the primary is always changing at different transformer ratio. Can somebody explain that?

Hmmmm. Is the secondary under load, with current flowing...in that case one has to consider resistor loss of the wire. But honestly, I have no idea what you are doing as an experiment. I am ondering if you are hooking up the secondary as a primary, then using the primary as a socondary with different ratio taps.