Where spell damage is not labeled “instant”, it is Damage Over Time (DoT). In that case, the value listed under “Damage” is not total/aggregate damage over the spell duration, but Damage Per Second (DPS). The total potential damage dealt by any DoT spell is “Base Damage Value * Duration”.
For example, Sonic Blast Level 6 deals 4657 blunt damage for 3 seconds, or 13971 blunt damage. A max level skull tower at 26,000 HP with a 100% weakness to blunt damage is destroyed by exposure to a full SB6, with 1942 blunt damage left over.
Weakness and Resistance values to specific types of damage will add a multiply or divisor to a unit/defensive structure’s base HP.
That said, descriptions aren’t always straightforward or strictly accurate, which can lead to confusion. A recent example is when the Snake Tower’s damage _specific to the hero alone _was buffed weeks ago. However, because it wasn’t a global buff (i.e., it didn’t apply to troops), the Snake Tower’s damage stat was left unchanged in the tooltip. As a result, the Tower’s DPS was selectively stronger, though it’s stats were not updated (this may have been rectified with yesterday’s update, btw).
If this tower’s capabilities and listed stats became skewed, it’s possible there are other disparities on towers, troops, spells, etc.