Why file name and uri of the file in database are different?
The reason for difference is that multiple files with same name can be uploaded in Drupal, so when you upload a file, it adds a counter at the end of filename so that if a file with same name is uploaded again, then it doesn't replace another one.

Also the filename column contains the exact name of the uploaded file as it is used while downloading the file (the file will be downloaded with this name).