What is my APK being signed with?

I have an android app project, on which I use the following command:

xbuild MyXamarinAndroidApplication.csproj /p:Configuration=Release /t:SignAndroidPackage

I got that from here.

I ran it and I get 2 APKs in my bin/Release directory, app.apk, and app-Signed.apk.

In my .csproj file I did not specify yet any of the android keystore properties:

<AndroidKeyStore>True</AndroidKeyStore>
<AndroidSigningKeyStore>public.keystore</AndroidSigningKeyStore>
<AndroidSigningStorePass>MyKeystorePassword</AndroidSigningStorePass>
<AndroidSigningKeyAlias>MyKey</AndroidSigningKeyAlias>
<AndroidSigningKeyPass>MyKeyPassword</AndroidSigningKeyPass>

But I got that signed apk anyway. How did that happen? Was a debug keystore automatically generated and used?

Thanks in advance!