Hungary refugee referendum invalidated

Low voter turnout has invalidated the referendum of Oct. 2 in which Hungarian citizens voted overwhelmingly to oppose any EU mandatory placement of refugees. The proposed plan sought to share 160,000 asylum seekers throughout the 28-member bloc through imposition of mandatory quotas. The Hungarian government had opposed the imposition of the plan, along with other countries. Prime Minister Viktor Orbán nonetheless expressed his support for the referendum's results as "excellent" and praised the turnout of voters.

From Jurist, Oct. 3 Used with permission.

Question on Hungarian referendum

Note that Orbán is asserting that more Hungarians voted against the quotas than the number who cast ballots in favor of joining the EU in 2003. Some 3.249 million votes were cast rejecting the quotas, compared with's 3.056 million votes in favor of joining the EU. (WP, Reuters) So why was the 2003 vote valid but this one not? A minimum 50% turn-out was required for the 2016 vote but not the 2003 one (which Wikipedia indicates received a 45.6% turn-out) ? Can anyone shed some light?