Why don't boosters double my attack?

The bonuses applied to the base attack (or health/recovery) stat are all additive, not multiplicative as most of us intuitively assume.

Using Dominion as an example: An event super may have 600% bonus, 55% bonus from lethal, 15% bonus from your zodiac leveling, 25% bonus from the zodiac boost on a tower and 100% if you activate a booster. The total bonus applied to the base attack stat then becomes 795%.

The booster doesn't double all the other bonuses unfortunately.

More details are available below in the screenshot taken from Pennypop's help page.

[Click image to enlarge]