Keith Black pistons VS Speed Pro for 318

Any of you guys with 318`s use the keith black pistons. i was looking on their web site and their calculator says that with the flat top pistons and my 58 cc heads that my compression ratio will be 10.3 to 1. and on the speed pro site it says with their flat top pistons that my compression ratio will be around 9.2 to 1. what is the differance in the pistions that can make that much differance in compression ratio.