This is related not only to starters, if you have many modules in your project they have different dependencies. As regular practice, there is a parent pom.xml with general settings.

If you see a dependency in child maven module like below:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
</dependency>

There is a configured dependency in parent pom like:

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>3.5</version>
</dependency>

Usually, it's more flexible to configure versions with properties:

<properties>
    <commons-lang3.version>3.5</commons-lang3.version>
</properties>

<dependency>
    <groupId>org.apache.commons</groupId>
    <artifactId>commons-lang3</artifactId>
    <version>${commons-lang3.version}</version>
</dependency>

MongoDb version is defined in spring-boot-dependencies using Dependency Management mechanism.

<properties>
    <mongodb.version>3.8.0-beta2</mongodb.version>
<properties>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.mongodb</groupId>
            <artifactId>mongodb-driver</artifactId>
            <version>${mongodb.version}</version>
        </dependency>
    <dependencies>
<dependencyManagement>

Dependency management is a mechanism for centralizing dependency information. When you have a set of projects that inherits a common parent it's possible to put all information about the dependency in the common POM and have simpler references to the artifacts in the child POMs.

Call mvn dependency:tree on your "other" project. Here you can see through which path you drew commons-lang. Joachim Sauer is probably right and it is a transitive dependency that you started to use like a direct one.

Best fix would be to start to use commons-lang3 for your project (again, Joachim Sauer is right here), second best fix would be to declare commons-lang as a direct dependency in your pom.xml.