Spells: Damage Over Time Spell

[Telin]

When I first cast a DoT spell on a target that I begin to kite, the damage is consistently reduced until either the target comes into melee range and either hits me or I cast another spell on them. For example, my dot was doing 8 damage instead of 13 until one of these actions were taken. I tested this repeatedly with the same result. They are able to partially resist dot spells I know, but this doesn't make sense.