The version 10.0.18 is considered bigger (i.e., more stable/final) than 10.0.18-dev
The -dev suffix usually stands for “development,” meaning a pre-release or unstable version.
In most versioning schemes (including Semantic Versioning and PHP's version_compare()), versions with a suffix like -dev, -alpha, -beta, -RC are considered less than the base version.
Hence Softaculous shows the upgrade option.
If you would like to prefer using the -dev version you can ignore the warning and wait for 10.0.19 version to be released by GLPI team.