Question about probability distribution in Chapter2.3

[myalos]

Say we get a reading of door, and suppose that testing shows that the sensor is 3 times more likely to be right than wrong. We should scale the probability distribution by 3 where there is a door.

If the reading is door and the sensor is 3 times more likely to be right than wrong, then P(door) should be 0.75 and P(wall) should be 0.25. Since there 3 doors and 7 walls in the map, it seems more reasonable that the probability distribution should be [0.75/3, 0.75/3, 0.25/7, .... , 0.75/3, 0.25/7]