Having played more, and having sent Missionaries to spread religion in distant cities, I have noticed that in a few cases, a very distant city with followers of my religion does not display any Pressure. But then later, when I spread religion to an intermediate city, linking them up, now both foreign cities display Pressure. So now I'm not sure if the Pressure displayed is some kind of amalgam of internal and external Pressure, or if there are some weird rules that we don't understand yet.The tooltip number you see is outgoing Pressure generated by the various populations of followers within the city. If external Pressure is being applied from nearby cities, you won't see any evidence of it in that city until a new follower of the external religion appears in the city. So, to find out what the external pressure is, you'll have to look at all cities within 10 hexes and add it up yourself.
Not the best UI... but that's Civ V for you.
It's a bewildering, obtuse, arcane, poorly-designed UI.
It does appear that the Holy City generates more Pressure than normal.