Why can I execute an Android application with which is compiled against API 17 on a phone running API level 10?

Why can I execute an Android application with which is compiled against
API 17 on a phone running API level 10?

I am building an Android application which I compile against API-17:
project.properties contains
target=android-17
I am slightly confused because I can download and execute the application
on an Android phone running Gingerbread 3.3.6, API 10. Can perhaps someone
clarify?