- GanonSmash
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- Exalted Heroic Member
The technical explanation would be because Bungie had the ability to use better graphics.
A canonical explanation could be that marines from different battle groups used different types of armor. The troops in Halo 1 were all on the P of A, which had a certain type of armor, while Earth's home fleet, including the In Amber Clad used a different style of gear.