The ineligible weeks have to drop out as they roll off the 16 week timeframe. So, if your ineligible weeks are all one right after the other and the 4 eligible weeks have been from the last 4 weeks, then you have 9 more weeks to go as they drop out each week. If you drop below 90, then you will add more ineligble weeks to the 16 week timeframe and you'll see no change.
If you have eligible weeks dropping out and you add another eligible week, then you will see no change.
There is a graph somewhere around here that explains it.