New construction in select target sdk and compile with doubts

I set the minimum SDK demo 7, target sdk 18, compile with 18, you can install and run a minimum of version 2.1, you can run the 4.0 version. There is a problem very puzzled looks SDK that goal does not seem to make sense? Is not only need to set the minimum SDK, you can install this version over a mobile phone (of course, the premise is no problem API)?
xml file,
android: minSdkVersion = "7"
android: targetSdkVersion = "18" />
And android: minSdkVersion = "7" />
For the installation to a different version of the phone, it has little effect?<-! Main posts under Banner (D4) -><-! Posts under the main text (D5) ->
Reply:
android: targetSdkVersionn is generally used to develop your version has been fully tested. The system does not turn on again to run the Compatibility Checker judgment in the application of this sdk version.