This happens because the total disk usage is never equal to the sum of all files there (which is what Nautilius shows once you select all files).
The reason for this is that the file systems themselves tend to occupy some space on the partition. Most likely, if you wiped out all data you store on that HDD, disk usage would be about 150GB. That space is reserved for the file system - it is required, as the file system need to store the data about files somewhere. ext4 pre-allocates this overhead space before any files are created, as opposed to - for example - ext3, where that space grows as more files are added to the partition.
If you consider these 150GB as a problem, please notice that it's just 5% of your total HDD size. If you would need more than 95% on you hard drive, you probably need to buy a larger one, instead of worrying about these 150GB that are out of your reach.
Also, please keep in your mind that in case of ext4 this space is not wasted. Data fragmentation is not a problem here, but the cost of this advantage is that extra occupied space. There are ways to decrease it and to force ext4 to use much less of this space, but that's not reccomended, as - because of fragmentation chances and other optimisations that won't be able to happen - this will very likely result in your machine working much slower, as data access won't be this smooth.