Why does maven reactor change the building order?

Question

I have a project with sructure like that:

./pom.xml - parent
./module-1/pom.xml
./module-1/pom.xml

./pom.xml - is an aggregation pom and a parent for ./module-1/pom.xml and ./module-1/pom.xml at the same time.

In this case, there is following building order:

[INFO] parent ............................................. SUCCESS [  0.332 s]
[INFO] module-1 ........................................... SUCCESS [  1.794 s]
[INFO] module-2 ........................................... SUCCESS [  0.165 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

but if ./pom.xml is just aggregation pom and it's not a parent pom for ./module-1/pom.xml and ./module-1/pom.xml than there is a following building order:

[INFO] module-1 ........................................... SUCCESS [  2.279 s]
[INFO] module-2 ........................................... SUCCESS [  0.363 s]
[INFO] parent ............................................. SUCCESS [  0.042 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

another one case:

./pom.xml - aggregation 
./parent-pom.xml - parent
./module-1/pom.xml
./module-1/pom.xml

parent-pom.xml - parent pom for ./module-1/pom.xml and ./module-1/pom.xml. pom.xml - aggregation pom.xml for ./module-1/pom.xml and ./module-1/pom.xml and parent for ./parent-pom.xml

[INFO] module-1 ........................................... SUCCESS [  1.953 s]
[INFO] module-2 ........................................... SUCCESS [  0.290 s]
[INFO] aggregation ........................................ SUCCESS [  0.058 s]
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESS

So I don't understand why does maven reactor change the building order?

I checked documentation and found the following rule:

Because modules within a multi-module build can depend on each other, it is important that the reactor sorts all the projects in a way that guarantees any project is built before it is required.

The following relationships are honored when sorting projects:

a project dependency on another module in the build a plugin declaration where the plugin is another module in the build a plugin dependency on another module in the build a build extension declaration on another module in the build the order declared in the element (if no other rule applies) Note that only “instantiated” references are used - dependencyManagement and pluginManagement elements do not cause a change to the reactor sort order.

But there is nothing here about parent-child relationship. Could anyone explain why the order is changing?