This can be for two reasons:
1. Typically a terminator character is added to the end of text files,
2. Granularity of the filing system.
Unfortunately, the size of your file fits a possible cluster size so it isn't easy to tell which it is. Clusters are the MINIMUM allocation size of a file. For example, if you saved a single byte file and cluster size was 4096 bytes, it would use 4096 bytes to store it. If the file was one byte bigger it would need two clusters even though the second one was almost empty so 8192 bytes would be saved.
The cluster size depends on the filing system used but essentially, it is the smallest 'unit' of storage. Every cluster has a number which is used to tell the filing system, through the directory, where things are stored so smaller clusters require more indexing numbers and hence more space taken up by the directory. There is a compromise between the size of the index (technically called a FAT or File Allocation Table) and the amount of space wasted in unused parts of clusters.
Sizes are always powers of two so 4096 (=2^12) is a likely candidate for the cluster size your OS is using.
Brian.