I think it is a result of using different size sensors, the Plus has a 4:3 aspect ratio sensor and the Pro a 16:9 aspect ratio sensor, so when you measure the
diagonal FOV for the sensor, it is longer on the Plus for the same visible width on the video image. Of course most people record in 16:9 aspect ratio on both cameras, so it is definitely confusing. Also the measurements are maybe of the lens FOV not the sensor FOV, and since they have different size sensors with the Pro being significantly larger in width plus only being 16:9 aspect ratio, it can use a narrower FOV lens to achieve the same horizontal FOV on the image! (The lens FOV has to cope with square sensors.)
In reality, the Pro 4K has the widest
horizontal field of view.
This doesn't include the Plus, but may be helpful, it gives an idea of the size of the differences. I think the Plus is similar to the standard A129, A119 V3 is narrowest, Pro is widest: