Active( online ) devices showing changes pending .
How can I see what changes has been made
Compare the config files of Active and Standby devices using diff comand in cli.
To see who made changes, goto /var/log/ and view audit
Is there a way to export configuration an compare in other editor
Yes, export those two config files to your desktop using cli or if you find that difficult, install WinSCP and copy the config file from /config/bigip.conf or if you have partitions, navigate to /config/partitions/<partitionname and compare them in Notepad++ (Plugins>Compare>Compare)
you can export configuration that hosted in /config/bigip.conf
In other way do you have this configuration in your F5?:
all this point above can modify configuration.
or try this command
show cm device-group <device-group> incremental-config-sync-cache