I've been told that he clearance on the bridges is posted as to the STRUCTURE; and, it doesn't include the hang-down lights, occasional maintenance "swings" or other similar temporary stuff, etc. Unless someone is reporting it differently, the sailboat's reported "vertical draft" is to the top of the mast without instruments, antennae, Windex, or flag staffs ;^)))). Also, the clearance may only be at the "center" of the span (a slight "arch" in the bridge isn't necessarily reported.)
Also, aside from tidal, wind, and flood run-off variations, you need to take into account a wake from other vessels that might influence you actual clearance.
I was told this by a fellow sailor who routinely takes his Swan through a bridge to the slip behind his house on the Severn River in Annapolis. He had a number of "tells" on the bridge piers, seawalls, etc. that he had to give him clues, warning, or comfort about what was the actual situation. The Federal Highway Administration, Corps of Engineers, etc. built a fixed bridge over the Severn just above the Naval Academy with 75' clearance. Just a short distance above that bridge, there is there is another fixed bridge which was built with 85' clearance. Go figure why they didn't make both the same???
We have 73' vertical draft to the "hard" mast on our J/Boat. I figure that there is at about 2' more to the top of the Metz antenna and other stuff up there. When I go under an 85' reported bridge, it looks like we're going to hit it from the deck.