"your device isn't compatible with this version"

Being that the 2 devices are on the same version the logical explanation that is causing the incompatibility message is that the devices are different and the app is using a feature that one of them doesn't support. Look at your AndroidManifest.xml and pay close attention to uses-feature and uses-permission tags. The Google Play store will use these to filter out devices that are not compatible.

For example if you have <uses-permission android:name="android.hardware.camera"> and one of the phone doesn't have a camera then it will be filtered out. Because the request for the permission implies that your app needs a camera to function properly. You can fix this by adding <uses-feature android:name="android.hardware.camera" android:required="false" /> into your AndroidManifest.xml. Of course, you'll also need code to safeguard your app from attempting to access the camera when it's not available.

Look at this page for more information, Google Play Filters.

Google Play has a way to indicate what's causing the device to be filtered out. It's very terse but go to the bottom of the page under devices and select Show Devices and then filter for Galaxy to verify that it's excluding based on manifest.

Double check the <uses-feature> tag in your AndroidManifest.xml file. It sounds like your application requires a feature that one of the devices doesn't support.

You should also check to see if you have the following declared in your AndroidManifest.xml,

<uses-sdk
    android:minSdkVersion="3" />

You want your application to work for devices running API 3 and above (not APIs 3 through 15). The code above will ensure that your application works with all devices above API 3. Note that you generally don't want to use the android:maxSdkVersion attribute (if you are currently using it).