search for: 505kb

Hello, i want to get the maildir current quota usage of an user, but i'd know what is the better way to do. in imap clients my quota usage, in example, show: 505KB, but if i sum only the size of each message from Maildir, i get total: 440KB, trying to use real and apparent block size but the sum differs from value showed in imap client. is quota usage based on why, have another way to do this? thanks