The top 5 topics for an epoch is calculated using all of your browsing history in that epoch.
A site observing a topic in the last 3 epochs unlocks the ability for document.browsingTopics() to return that topic from your top 5.
Every epoch each site has a 95% chance to be assigned 1 topic out of your top 5 topics and a 5% chance that it is assigned a random topic. When browsingTopics is called it gets the topics it was assigned for the last 3 epochs. Real topics are not returned if the site did not observe that topic in the last 3 epochs as mentioned in the previous paragraph.
A site observing a topic in the last 3 epochs unlocks the ability for document.browsingTopics() to return that topic from your top 5.
Every epoch each site has a 95% chance to be assigned 1 topic out of your top 5 topics and a 5% chance that it is assigned a random topic. When browsingTopics is called it gets the topics it was assigned for the last 3 epochs. Real topics are not returned if the site did not observe that topic in the last 3 epochs as mentioned in the previous paragraph.