Why are the state variables passed into the filter not equal to the state variables defined by each [sensor]_config matrix?

edit

I am trying to get a deeper understanding on how ekf_localization_node works.
At the moment I am stuck in interpreting the output of the debug file.

In particular, from the output of that file, I am trying to understand what the line Update indices on the debug file indicates:

Case 1:
Measurement topic name is:
- odom0_twist

Update indices are:  
 [2        3        4        6        7        8        9        10       11       14       ]

[EDIT 1:] Case 2:
Measurement topic name is:
- imu0_acceleration
- imu0_pose
- imu0_twist

Update indices are:
 [2        3        4        8        9        10        11        14        ]

From the source code file ekf.cpp I get that the filter checks which measurement state variables actually get passed into the filter. Only if a state variable value is not 'nan' or 'inf' the index is pushed back into updateIndices, which is then printed to the debug file.

I assume that the index value refers to the order/index of the state vector elements. And from the documentation I get the impression that the state vector elements are ordered as follows:

0: position x
1: position y
2: position z
3: rotation roll
4: rotation pitch
5: rotation yaw
6: velocity X
7: velocity Y
8: velocity Z
9: velocity roll
10: velocity pitch
11: velocity yaw
12: acceleration X
13: acceleration Y
14: acceleration Z

Therefore I conclude that the line Update indices from the debug file is telling me that following state variable are passed into the filter: 2:position z , 3:rotation roll, 4:rotation pitch, 6: velocity X , 8: velocity Z, 9: velocity roll etc..

This does not match with the [sensor]_config matrix (a.k.a true/false matrix) values that specify what variables should be fused into the final state estimate (see configuration file below).

Could some help me to understand what the line Update indices on the debug file indicates?

[EDIT 2:]

Code comments confirm that 'Update indices', in effect, describes which state variables that are passed into the filter.

The question that remains is:
Why are the state variables passed into the filter not equal to the state variables defined by each [sensor]_config matrix?

# My configuration for the robot_localization EKF node
frequency: 10

two_d_mode: true
debug: true
debug_out_file: /home/rob/catkin_ws/src/ekf/ekf_debug_file.txt
publish_tf: false

odom_frame: odom
base_link_frame: base_link
world_frame: odom
# map_frame: map


odom0: /odom_noisy
odom0_config: [false, false, false,
               false, false, false,
               true,  true,  false,
               false, false, true,
               false, false, false]


odom0_differential: false


imu0: /mobile_base/sensors/imu_data_raw

imu0_config: [false, false, false,
              false, false, true,
              false,  false,  false,
              false, false, true,
              true, false, false]

imu0_differential: false