1500/256kbs and 8000/384kbs - I need some help here!?!?

OK, I recently upgraded from a 1500/256kbs internet plan to a 8000/384kbs but my download speed remains unchanged. Even when I run speed tests nothing at all has changed. All the results are exactly the same. So what exactly do the numbers at the front mean (1500 and 8000) because I'm a little confused. Thank you.