Spot fleet launches Spot instances in the lowest priced Availability Zone

Starting today, Spot fleet automatically launches new Spot instances into the lowest priced Availability Zone (AZ) when you specify multiple VPC subnets or AZs in the Spot fleet launch specifications. Previously, when you required Spot fleets in the cheapest among a specific set of subnets, you had to determine the AZ with the lowest Spot price and request your Spot fleet in the corresponding subnet. 

Source: Amazon Web Services News