Each of those were done to Cerberus from full HP (one swing perhaps). The range is quite exaggerated for a single swing. That also doesn't explain why some Spirits Within are receiving a positive bonus while others receive a negative bonus.
Looking over the logs:
Code:
[21:24:51]Yugl uses Meikyo Shisui.
[21:24:53]Yugl's Meikyo Shisui effect wears off.
[21:25:03]Yugl starts casting Stoneskin on Yugl.
[21:25:13]Yugl casts Stoneskin.
[21:25:13]Yugl gains the effect of Stoneskin.
[21:25:18]Yugl uses Third Eye.
[21:25:22]Cerberus is out of range.
[21:25:22]The Earth Elemental starts casting Enstone.
[21:25:26]Unable to see Cerberus.
[21:25:26]Yugl hits Cerberus for 5 points of damage.
[21:25:27]Yugl readies Spirits Within.
[21:25:29]Cerberus misses Yugl.
[21:25:29]Yugl uses Spirits Within.
[21:25:29]Cerberus takes 984 points of damage.
Code:
[23:13:46]Yugl uses Meikyo Shisui.
[23:13:48]Yugl's Meikyo Shisui effect wears off.
[23:13:56]Yugl starts casting Stoneskin on Yugl.
[23:14:02]Yugl's casting is interrupted.
[23:14:04]Unable to use job ability.
[23:14:07](2*) You must wait longer to use that ability.
[23:14:07]Yugl uses Third Eye.
[23:14:11]Cerberus is out of range.
[23:14:12]The Hilltroll Red Mage starts casting Blink.
[23:14:15]Yugl is drawn in!
[23:14:16]Unable to see Cerberus.
[23:14:16]Yugl hits Cerberus for 11 points of damage.
[23:14:17]Yugl readies Spirits Within.
[23:14:19]Cerberus misses Yugl.
[23:14:19]Yugl uses Spirits Within.
[23:14:19]Cerberus takes 839 points of damage.
Code:
[23:26:08]Yugl uses Meikyo Shisui.
[23:26:09]Yugl's Meikyo Shisui effect wears off.
[23:26:09]The Hilltroll Red Mage starts casting Enfire.
[23:26:14]The Hilltroll Red Mage casts Enfire.
[23:26:14]The Hilltroll Red Mage gains the effect of Enfire.
[23:26:21]Yugl uses Third Eye.
[23:26:22]Cerberus is out of range.
[23:26:26]Yugl is drawn in!
[23:26:26]Yugl hits Cerberus for 11 points of damage.
[23:26:27]Yugl readies Spirits Within.
[23:26:29]Yugl parries Cerberus's attack with his weapon.
[23:26:30]Yugl uses Spirits Within.
[23:26:30]Cerberus takes 815 points of damage.
Those are the 1970HP Spirit Within tests according to the damage I posted and the logs. Two of them have the same HP taken and the damage done increases as his HP increases (Opposite of the effect you posted).