Talk:Void Relic/ByMission/@comment-33634509-20180118125504/@comment-75.35.182.132-20180905023203

It has to do with the level of the enemies:

Enemy tier:
 * Tier 1: Level 1 - 25
 * Tier 2: Level 25 - 45
 * Tier 3:: Level 45 +

http://warframe.wikia.com/wiki/Defense#Endless_Defense_Missions