Yes that is because the booster adds fermentables. I am not familiar with boosters but would suggest altering a recipe or getting a kit that is designed for a higher ABV. The result of just adding something is usually to thin out or dry out the beer. It will change the beer from what it was intended to be.
Wonder why it was included as an option, then?
Since it will change the flavor of the recipe...
A booster is usually a mixture of sugar and Malto-dextrin. It is flavorless, and will not thin out the beer. It's fermentability is similar to extract. It is actually a pretty good product for increasing the abv with minimal impact on the balance of the base beer.