y, you're right. it depends on your base p.atk. your logic is almost correct, but you're forgetting to remove all other % bonuses before you multiply x(1.05) = y
It's not adding 5% because you have static p.atk bonuses be it through passives or items so start looking for what static bonuses you have. By your name, I assume you're a summoner so you can start with this.
Superior Weapon Mastery
Increases INT by 2, P. Atk. by 82.9, M. Atk. by 103.7, Casting Spd. by 23%, M. Atk. by 6% and M. Critical Rate by 6%. Decreases MP Consumption of magic skill by 6%. When a Sigil is equipped, increases M. Atk. by 5% and MP Recovery by 20%. Additionally, there is a chance the Cooldown of skills will reset or the duration double depending on your INT. When equipped with a two-handed blunt, additionally P. Atk. + 105% and P. Accuracy + 4.