Quote from SiameseCookie »
How is health more effective if he gains an extra percentage on his resistances? More resistances means more extra resistances from that passive.
It's about getting the most effective health with the least gold. Since you get free resistances, you can increase your tankiness in a "cheaper" way by building health first.
In a way, yes. But this is how I see it: Consider that health would have the same cost as resistance per effective health. Then buying resistance would give you more effective health since you gain an extra percentage of it for free. However, health is cheaper than resistance at the moment. So health might be more cost effective on Garen because it is so cheap, and not because of his passive (which gives more cost-effectiveness to resistances).