They changed this with update 24 so that spell or physical resistance now applies a lower value to both. So, say 2800 or so spell resist or 2800 physical resist now both provide 1400 or so armor.
Not great if you have a lot more of one or the other due to, say, being a dragon knight. DKs have a passive that increases spell resist, so getting 2800 physical resist is better for that character than 1400 applied to both. But, oh well...