It's just the way RAM is advertised, vs the way it is calculated. Generally, they round up when advertising, or use some combination of 1000 bits to a byte, 1000 bytes to a megabyte, 1000 megabytes to a gigabyte.
When you view it via your hardware (be it phone or computer or whatever), it calculates it the proper
way, where there are 1024 bits to a byte, and so on. So what you see on your device is almost always lower than what was advertised. This is pretty common with drives and memory, and the larger the storage, the more extreme the disparity becomes (my "2 terabyte" drive is actually only 1.81 terabytes, for instance, I've "lost" almost 200Gb).
The method in which the storage is formatted also has an impact, there's some overhead for this which uses space, once again the larger the storage, the more that is wasted/lost for this overhead.
Long story short, there's nothing wrong with your Milestone.