After downloading the SRX configuration from the Mist UI and comparing it with the output of the following command on the SRX device:
show configuration | display set | count

Administrators may notice that the total number of configuration lines in the downloaded file does not match the count reported by the SRX CLI.

For example:

SRX CLI reports higher number of configuration lines using " show configuration | display set | count "
The configuration file downloaded from Mist would have less lines. 

This behavior is expected.

As part of SRX management, Mist automatically installs and maintains certain operational configurations on the device, including items such as:

event-options
system scripts
mist-related management configuration
other system-generated configuration required for cloud management

These auto-generated Mist-managed sections are not included in the configuration downloaded from the Mist UI because the downloaded file represents the configuration intent maintained within Mist rather than the complete device configuration.

The Download Configuration option in Mist provides the intended configuration managed by Mist, not the complete running configuration present on the SRX device.

Mist-generated and auto-managed configuration elements are not included in the downloaded configuration.