Er..... no. 512Kb/sec is .5 Mb/s, and 1200Kb/s is 1.2 Mb/s - (1000 kibibbytes in a Mibibbyte, or 1,024 Kiilobytes in a Megabyte!)
I read the OP's post as if he was getting between 512 and 1200 kilobytes per second transfer rate and wondering why it didn't come up to the ISPs quoted speed of 8 megabits per second.