It's a bit of a silly comparison considering
1. Halo's decline as a system seller
2. Halo 5 isn't exactly a mainline Halo game
3. Install Base difference
4. Vgchartz don't seem too accurate in XBO game sales. Why I don't know.
It's a good achievement for UC4, but i doubt Uncharted 4 will ever outsell Halo 3 or 4.