Two things. You want to make sure that you are getting dedicated and not shared bandwidth. If you are subscribed to shared bandwidth, then you may not get the "claimed" amount. If you find that you have dedicated bandwidth, and you aren't getting what you subscribed to, then you need to look into your SLA from the carrier.
If you haven't purchased bandwidth yet, then be sure to look into the SLA and read the disclaimers before you buy, so that you have a clear understanding about dedicated vs. shared and what the carriers commitment is to you if you aren't getting the amount you subscribed to.
No company provides the exact bandwidth as they mention in their banners. And Iam sure we will get only the 75% of the bandwidth, and other will be reduced because of many factors.
Make sure you know how your SLA is worded. Most will not even give you a throughput SLA, so demand it if you have negotiating power - then make sure you specify how it is calculated. If you have dedicated service, the carrier will just point to the port and say, "you are getting the full bandwidth."
is the achieved bandwidth is same as claimed?
