Well as an example, say there is an attacker and defender both with a base strength value of 2.0. If the attacker had a +10% attack promotion, it would reduce the defender's strength to 1.8 and the attacker would still be 2.0. However if the defender also had +10% defense (from terrain for example) it would raise its defense to 2.0 again. Compare this with what you might expect, where both attacker and defender would have a modified value of 2.2.
Basically all combat modifiers, whether they be from promotions or terrain, add or subtract to the defender's base strength. You add up all the modifiers to get the overall defense modifier and then apply it to the defender.
The Combat (1 to 6) promotions are the only exception, as they always modify the strength of the unit who owns them rather than strictly the defender.