It seems the only difference is the number of decimals.
If you do not specify how many decimals (you can in legend tab), then it’s going to use the same number of decimals as the graph Y axis, and that is determined based on the min/max of all series rendered. When you only show one series the min/max could change (as only one series is being renderd) and hence the decimals used for the legend can also change when you click to only show one series-