Reproduction Steps:
1. Open User's project
2. Make builds resizable (PlayerSettings > Resizable Windows)
3. You can use Default Screen Width/Height 1280x800 (but it's not working as expected)
4. Build for Standalone
5. Choose Resolution 1024x768 and windowed mode
6. Build/open one more standalone
7. Make Resolution 1280x800
8. If not using Default Screen Width/Height resize it manually
9. Note to make it easier you can add Text field and script with line :
"GetComponent<Text> ().text = Screen.width + " x " + Screen.height;"
10. Compare First and Second builds on Retina Display
11. Notice that text quality on Second build is worse than on First build

Reproduced on versions: Unity 5.3.5f1, 5.3.6p6, 5.4.1p1, 5.4.1p2, 5.5.0b5