Each of the islands (allods) has its own magic and its own laws of physics. In other words, on each server – magic, substances and materials of weapons and armor (steel, mithril, adamantine, etc.) works differently.

The table shows which material is relevant on which allod

Allods (server) number Material
#2 easy leather and bronze (iron is rare)
#3 medium iron, steel and mithril (rare adamant)
#4 medium mithril and adamant
#5 medium adamant and meteor
#6 hard meteor and dragon leather
#7 horror+ crystal

Note: chain and open armor weighs significantly less than full armor.

Also… Leather armor has increased protection at server #2/3, although this is not written in the description of a particular item (check your character sheet).