Seldaek/monolog — comment on commit f13a538b41eb98e4a1d104534e8e037dcd62172c
File: composer.json line 6
By stof:

the version is optional in composer.json and is guessed based on the tag name. the goal is to make it easier for devs: they don't need to remember about composer when releasing a new version.
Note that you can still use the version explicitly to force a different one (for instance if you don't follow the supported convention to name your tags)