Some % of the population buys more than one mattress in a 3 month period, intentionally. Perhaps they are landlords, or have 2nd homes, or want to stock their new home with new furniture.
Maybe, but (thank goodness) the mattress ads bombarding does not last 3 months, only a week or so, after 2 and 3/4 months have passed that % of population will either look where they bought the previous one (if they are satified with it) or make a new google search, in both cases the week long targeted campaign won't have much effect.