Random numbers for example. There are 1000 active cities in the event, that means at 1%, only 10 can be in Gold at one time. As soon as the first 10 cities earn any points at all, then Gold is full, and you have to earn more points than the lowest scoring of those 10 in order to take their place. The only time this is not true is during the final hour of an event, when the minimum needed for each league is frozen. At that point, more than 10 of those 1000 could make gold and not kick anyone out, because the percent limit is lifted for that last hour. Other than during that final hour, if you are on the edge of what is needed for 1% and gold league, and someone else increases their score to just higher than yours, you drop to 1.001% and fall out of Gold.