The idea is that you should still be able to make patch releases to the version compatible with the previous Julia version. For example: Package A@1.2.0 is compatible with Julia 1.0 (current LTS). If you release A@1.2.1 with Julia compatibility raised to 1.5 for example, what do you do when a user stuck on Julia 1.0 finds a bug in A@1.2.0? The 1.2.1 version number needs to be reserved for that case.
Edit: Just to add to the above; the package manager handles it just fine, so this is just a policy for the General registry that is meant to stop package authors from shooting themself in the foot like in the example I gave.