by AL Rachels.
I think you have just discovered a bug. It was changing the Activity completion that reset the max to 100. Just checked on my site and get the same problem. If you go back and set the Max grade to what you had it before, be sure you select NO for, Rescale existing grades, or the current ratio will be maintained and you will have go manually reset the grades.
And by the way, 100 points is a default setting on the Site administration > Grades > General settings page.