This is due to the value of the serialize_precision PHP setting.

Inputting a decimal value, in something like a CSS value, can be saved with multiple decimal places rather than what the user input. The value displays correctly in the UI, but when exporting the configuration the value has multiple decimal places, this is due to the serialize_precision PHP setting. This can cause unnecessary conflicts with configuration when exported and committing to code.

It is recommended that environments where configuration is synced that the same serialize_precision value is set.

The PHP default value of serialize_precision prior to 7.1.0 was 17, on later versions of PHP the default is now -1. The value of the setting can be seen in your Drupal sites Status report. It can be overridden in your settings.php or php.ini.