Armenia voted against the UN resolution on the illegitimacy of the Crimean referendum because of its national and foreign policy interests, since the country has always voted in favor of the right of nations to self-determination.  
Andranik Manukyan, Armenia’s Ambassador to Ukraine, stated the aforementioned.

“We have a Karabakh issue, where a referendum on independence was conducted in 1991. The Nagorno-Karabakh referendum was held in keeping with the Constitution of the ex-USSR.

“Ninety-nine percent of the referendum participants had voted for independence. And from that moment, Armenia proves to the entire world that this is a legitimate decision, since the right of nations to self-determination—which is written in the UN Charter—is not simply a precept of international law, but rather its objective,” Ambassador Manukyan told Segodnya.ua news agency of Ukraine.

“Armenia voted against the [UN] resolution on Ukraine not that we are against Ukraine. Our country has not betrayed Ukraine. On numerous occasions, Ukraine has supported—with its vote—the Azerbaijani party, in the settlement of the Nagorno-Karabakh conflict. But, on this occasion, we [Armenia] did not send even a protest note to the Ukraine MFA,” Andranik Manukyan also said, in particular.