no not naive and stupid at all! but the price advertised is an asking price - it's a guide as to what the vendor thinks its worth.
perhaps they negotiated?
or if they could move in quicker they were offered a discount?
or did you buy a few months later when the site was nearer to completion?
there's soooo many factors that could affect the purchase price, there's really no point worrying about it. i paid the absolute ceiling for my house and at the time i was narked, but i loved the house so it didn't really matter. were their sales around the same time if it was new build? if so, there's nothing really you could do (land registry takes 3 months to list a sold price), but there's no harm in researching in future. nearby comparables, local agents opinions etc. if you bought off plan then someone buying when they're completed will pay more again to what you paid.
if you love the house, don't worry about it, just enjoy it!