See below image. This is the lens that is shown with missionaries, etc.
One would think that the numbered overlay shows the number of citizens in the city adhering to each religion. Aachen thus has 0 Confucian citizens (green) and 6 Absurdism citizens (blue - my religion). However, this doesn't line up with the City Details dialogue where it shows 0 for Confucianism but doesn't list Absurdism at all.
That's only the case for Aachen, which also happens to be the only city with a Holy Site. Aachen is also the site where I used a Great Prophet to found the religion. For the other two cities, the Religion view lines up with City Details. So perhaps this part is just a bug in City Details?
One thing that is clear: The circles are just a graphical representation of the proportion of citizens for each faith. At first, I thought it might be a progress indicator, but it isn't.
One related issue: Sometimes when I spread faith with a missionary, the citizen count doesn't move at all. For example, I spread on Berlin but it stayed at 3. Is this a rounding error or does spreading faith not always work?