Don't worry about it... has to do with the filesystem's blocksize and how it stores information. Basically, think of like gallons of water vs liters of water. If you have a certain amount of water to store, it won't exactly fill the gallon or the liter probably. If you multiply the total number of liters needed by the volume of a liter and the total number of gallons by the volume of a gallon, you get different total volumes. However, you have the same amount of water.
File size reported is the amount of space it takes on that computer using that filesystem, not the actual amount of useful data. Try copying files between filesystems (like ntfs to ext3 or something) and they will often change a bit.