Your RAM is detected correctly. MB (megabyte) and MiB (mebibyte) are two different units.

• 1 MB is 1000 KB which is 1000 byte (decimal)


• 1 MiB is 1024 KiB which is 1024 byte (binary)

If you check the math yourself you will see that 512 MB are 495 MiB ;-)

MB is mostly used for selling stuff because it's the higher number. In your Ubuntu system you will mostly encounter MiB (or GiB).