move_base's obstacles don't match laser scan output

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I have a (physical) robot with hokuyo laser mounted. I use rviz to see the raw scans. I also run move_base and subscribe to /move_base_node/local_costmap/obstacles on rviz to see the cells marked as obstacles by move_base.

One phenomenon I notice is that depending on the minimum obstacle height in costmap_common_params.yaml, move_base either sees no obstacles or sees obstacles in every cell. What I expect on rviz is that move_base marks a cell as an obstacle precisely when the laser scan indicates there's an obstacle on the cell. I have been trying to fine tune min_obstacle_height to this. Below 0.21 m, move_base marks every cell as an obstacle; above this, it sees no obstacle at all!

I don't know how to deal with this problem. My costmap_common_params file is:

map_type: costmap

laser_scan_sensor: {sensor_frame: laser, data_type: LaserScan, topic: /scan, expected_update_rate: 0.2, observation_persistence: 2.0, marking: true, clearing: true, min_obstacle_height: 0.20, max_obstacle_height: 2.5}

Update after tfoote's comment

Screenshot of rviz with obst height = .21 m :http://tiny.cc/a96op

Screenshot of rviz with obst height = .20 m: http://tiny.cc/yv4k5

Please see the captions of the images for more detail.