Android launch mode issue

Question

Although I have been searching on Stack Overflow for quite some time, did not find the answer I want so far.

1. Problem description:

• I have an Android app with bunch of activities and submitted to Google Play. After it's published, I downloaded it, whenever open the app, it starts a new task. I need to click the hardware back button several times to kill all the tasks. While, after that, whenever I launch the app again, everything is fine.
• Another weird thing is that there's no problem if I directly install the app on the device through adb command (adb install xxxx.apk)

2. What I did:

• originally, in the code, I was using the "standard" launch mode for the launch activity. I encountered the problem as described above.
• after I read the official document, I realized that "standard" mode will launch an instance of the activity everytime, so I changed the launch mode to be "singtop", but the problem is still there.

3. What I do not understand:

• Why the app keeps creating new task if I install the app from Google Play. Why "Singtop" launch mode does not work in this case?
• why I do not have the problem when install the app directly through adb command, while I get the problem when install app from Google Play. What's the difference.

4. Source code for reference:

• build.gradle (Module:app):

  apply plugin: 'com.android.application'
  
  android {
      compileSdkVersion 23
      buildToolsVersion "23.0.1"
  
      defaultConfig {
          applicationId "com.example.testing.exampletesting"
          minSdkVersion 17
          targetSdkVersion 22
      }
  
      buildTypes {
      release {
          minifyEnabled false
          proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
          }
      }
  }
  
  dependencies {
      compile fileTree(dir: 'libs', include: ['*.jar'])
      testCompile 'junit:junit:4.12'
      compile 'com.android.support:appcompat-v7:23.1.0'
  }
  

• AndroidManifest.xml:

  <?xml version="1.0" encoding="utf-8"?>
  <manifest xmlns:android="http://schemas.android.com/apk/res/android"
      package="com.example.testing.exampletesting"
      android:versionCode="10"
      android:versionName="1.0.1" >
  
  <application
      android:allowBackup="true"
      android:icon="@mipmap/app_logo"
      android:label="@string/app_name"
      android:supportsRtl="true"
      android:theme="@style/AppTheme" >
      <activity
          android:name=".LaunchActivity"
          android:launchMode="standard">
          <intent-filter>
              <action android:name="android.intent.action.MAIN" />
  
              <category android:name="android.intent.category.LAUNCHER" />
          </intent-filter>
      </activity>
  
      <activity android:name=".DummyAActivity" />
  
      <activity android:name=".DummyBActivity" />
  
      <activity android:name=".DummyCActivity" />
  
  </application>
  
  </manifest>