Android : Google play store automatically setting max api level for a new apk

I uploaded a new apk to the play store, setting the target and min sdk levels in gradle like this:

android {
compileSdkVersion 21
buildToolsVersion '21.0.2'

defaultConfig {
    applicationId "com.myapp"
    minSdkVersion 10
    targetSdkVersion 21

    versionCode 6
    versionName '1.5'

    testInstrumentationRunner "android.test.InstrumentationTestRunner"
}

But now the pla store developer console reports that this apk is available for api 10-18 and the play store reports:

Requires Android
2.3.3 - 4.3

AndroidManifest.xml does not contain any reference to api level.

Why is google play automatically setting 18 as the max sdk level?