When Ifrit used eruption in phase 3, the animation had a momentary build-up before the explosion. Because of this momentary buildup, there was a slight delay between the server's check and the explosion. This has already been corrected for phase 4 and release.
1. Registering Player Damage
In phase 3, when judging whether a character was standing in an attack's area of impact, your position was checked using a circle around your character to represent the space your character stands in.
Furthermore, to keep the different races balanced, the radius of this circle was the same for all characters. Because of this, even if you appeared to have barely escaped the area of effect, a portion of the circle was still within range. This created the illusion that you barely evaded attack, with varying results depending on your race.
For phase 4 and release, this circle has been reduced to a single point at the center of where your character is standing to more accurately determine your position.
2. Enemy Progress Bars and Area of Impact
Being an online game, there will always be a minimal delay when input is received from the player and sent back after being processed by the server.
In FFXIV: ARR, your position is checked by the server once every 0.3 seconds. This timing is synced and processed according to the servers to prevent lag between your input and what you see.
During phase 3, however, this syncing did not take place due to stringent server checks that took place, causing lag between what you saw on the enemy progress bar and your character's positioning. As a result, even if you appeared to be outside the area of impact when the progress bar was filled, damage was still taken.
Adjustments are being made to reduce lag as much as possible, and syncing should improve the timing between enemy cast bars, area of effect markers, and the actual attacks.