AC is a combination of two components, your avoidance and your mitigation.
The AC number you see on gear only counts towards mitigation. If you add 1000 AC to your gear, you will not take a single less hit than you would have taken with absolutely no gear at all. The advantage of gear AC is that those hits will tend, on average, to be for less damage. So with 1000 AC worth of gear, you're much more likely to survive taking those hits than you would have naked.
AGI affects your avoidance value. When your AGI drops below 75 (due to excess weight of gear or detrimental effects cast upon you) you take more hits. As your AC goes up from 75, your avoidance increases, but not as quickly as it in effect increased getting to 75.
Now it is possible that if your AGI dropped below 75 the AC value you see listed on your character did not appear to decrease all that much. That's because the bulk of the displayed value is from your mitigation AC. In that case, you will rapidly see mobs' accuracy increase (and thus your health drop faster and faster in fights) even while the AC value appears to have only dropped slightly.
Bottom line, you should always try to keep your AGI above 75. While the apparent change to your displayed AC is slight, the actual impact of the change can be quite severe as mobs go from landing (as an example) 50% of their attacks to 80% or more...
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