MySQL Collation utf8_unicode differences
Unicode collation names may include a version number to indicate the version of the Unicode Collation Algorithm (UCA) on which the collation is based. UCA-based collations without a version number in the name use the version-4.0.0 UCA weight keys. A collation name such as utf8_unicode_520_ci is based on UCA 5.2.0 weight keys.

See https://dev.mysql.com/doc/refman/5.6/en/charset-collation-names.html.