When you are selecting the Default option in Image > Adjust > Threshold the threshold value is computed automatically based on the image histogram of the currently active slice of your stack. You will notice the latter when you close the Threshold window, change to a different slice in your stack and reopen the Threshold: the value will (most likely) have changed.
If you really want to set the same threshold for all the groups, is specifing it manually an option? When you click the Set button in the Threshold window, you enter values that are used for thresholding (see https://imagej.nih.gov/ij/docs/guide/146-28.html#sub:Threshold...[T] for a more detailed explanation).
Also, take a look at this page on the ImageJ wiki - it might be useful: