robot_state_publisher fixed frames 0.5s ahead

Hi guys, The **robot_state_publisher** has 2 different mechanisms for publishing **moving** and **fixed** frames. Moving frames are published on **joint_states** callback. Does anyone know why **robot_state_publisher** publishes the fixed frames using a timer independent from the **joint_states** subscriber and ahead of 0.5 seconds? Could not find any valid reason whether from the internet or the code... Thanks ;)