You are forgetting that the value of a dollar can (and does) fluctuate just as easily as the value of a Bitcoin...
http://disposablewisdom.com/wp-content/uploads/2012/03/173-1214130656-fed-purchase.jpg
http://disposablewisdom.com/wp-content/uploads/2012/03/173-1214130656-fed-purchase.jpg
Luckily bitcoin's value doesn't decrease with time but it has an exponential growth which hasn't stopped yet
