IMU data to be used with robot_localization

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I am using the package robot_localization with an IMU. In the docs, the Preparing Your Data for Use with robot_localization, it says

Adherence to specifications: As with odometry, be sure your data adheres to REP-103 and the sensor_msgs/Imu specification. Double-check the signs of your data, and make sure the frame_id values are correct.

REP-103 says the axes should be oriented in the following fashion:

• x forward
• y left
• z up

Which means when an IMU is placed in its neutral position (flat on the surface), the axes should look like this:

So for acceleration due to gravity, it should measure - (minus) 9.8 meters per second squared for the Z axis. However the docs say:

Acceleration: Be careful with acceleration data. The state estimation nodes in robot_localization assume that an IMU that is placed in its neutral right-side-up position on a flat surface will:

• Measure +9.81 meters per second squared for the Z axis.
• If the sensor is rolled +90 degrees (left side up), the acceleration should be +9.81 meters per second squared for the Y axis.
• If the sensor is pitched +90 degrees (front side down), it should read -9.81 meters per second squared for the X axis.

This would mean the axes are oriented in the following manner, implying left handed coordinate system:

I am definitely missing something here. Can anyone help?