The weekly MHO calculation is based on the percentage of opinions that got MHO out of the amount of questions you have shared an opinion on where MHO was selected. This means that if you have shared an opinion on a question where MHO has not been selected yet, that opinion does not lower your MHO %.
I have a feeling it is something to do with the differences between the opinions total in your MHO percentage and your opinion shared total on your profile or in your ranking profile. I know anon opinion are one difference but what is second difference. It is nearly 2,000 for me.