Relics you get depend on the size of the hit(s) (the amounts of units lost by all the players engaged in the battle(s)). If you hit a heavily defended Presidio with 100M off, you're bound to get a better relic. If you keep sending roughly the same amount of units to prizes, you'll keep getting the same relic (and the one you showed is a very common one to get there).
In short: it's a feature.