Might be an incorrect sign. It does happen. A road gets paved and now is a couple inches higher and nobody bothers to change the sign. Any new bump, even one not directly under the bridge, can cause a collision. We cannot expect drivers to get out and measure every underpass ... thats what the signs are for!

Or look up 11foot8 videos and watch a multitude of dingbat drivers rip the roofs off their vehicles despite correct signage and a flashing warning.