Partition And Missing Data - New HD

I will use this partition as example. I made 50GB partition on my new hard drive why does it only show 49.9GB instead of 50GB? It’s small amount of missing data, but would like to find out what is happening. The same situation also applies to my other partitions on my new hard drive.But when I look at my old hard drive partitions using My Computer / Properties which is attach to the same computer it gives me correct amount of data on the partitions. Example would be 100GB not 99.9. These partitions on my old hard drive were setup when I was installing Windows XP several years ago.