You're not mistaken. The problem is that the multiplier is only part of the equation.
Simplistically, suppose we use your example of Preserver. Her A1 has an 11x ATK multiplier. However, her base ATK is only 771. Also, assume she has her A1 fully booked, giving her an additional 10% more damage. Again, simplistically, that gives her 11 * 771 * 1.1 = 9329.1.
Now, compare that with someone Candraphon, a more traditionally renowned A1 smacker. His A1 is "only" 6x ATK (assuming the enemy has a buff). However, he has a base ATK of 1509, and his A1, fully booked, has an additional 30% damage. That gives him 6 * 1509 * 1.3 = 11770.2, which is 26% more damage than Preserver gives.
Ontop of that, because his base ATK is significantly higher, his ATK% items give much more bang for their buck. Suppose we use an item like an ATK% chest, giving 60% more ATK. For Preserver, that yields 771 * 0.6 = 462.6 bonus ATK. For Candy, it's 1509 * 0.6 = 905.4 bonus ATK, which is almost double what Preserver gets.
Consider that most people will have a *lot* of ATK% bonus on their items - 60% from chest, and usually another 100% from substats and glyphs. That adds up to a major differentiator.