This difference is caused because one tool counts only file sizes, and the other counts real space occupied by these files and directories.
Here you can read about "clusters":
https://en.wikipedia.org/wiki/Data_cluster
In short, when you have 4-kilobyte clusters on your system (and you probably have), then even 1-byte file occupies whole cluster, which is 4 kilobytes. This article will further explain this difference.