Why is the effective hard drive size lower than the actual size?

Because NTFS by design uses a piece of your volume for a MFT (Master File Table) which holds file names, creation dates, access permissions, and contents as metadata. The bigger the volume, the bigger the chunk NTFS will need.

Actually. This is because harddrive manufacturers conform to the 1MB = 1,000 KB rule and your OS uses the 1MB = 1,024 KB rule.

This would probably account for most of your lost space. Its normal. You will NEVER buy a drive that is the size it specifies, ever. I looked in to it myself not too long ago :)

Unfortunately, life's hard and this is one of those things that annoys people non stop.

At least you are smart and understand that there is a conversion that needs to be made.

The "extra" space is usually used up by the allocation table, meta data and various other items that just make it work without you needing to think of it... it is nothing to worry about.

If you want to make 100% sure you have the correct drive, go in to device manager and expand hard drives, find the model number and Google it. If it reports the correct drive (which it should) there shouldn't be anything to worry about.