Well, first of all the official Nazi numbers of 99,75% are bollocks.

This is the ballot card used. Additionally, voting wasn't done in a private booth like you'd expect it, but right in front of the people who handed you the paper, probably flanked by Wehrmacht or SS soldiers. Very few would've had the guts to openly defy the Führer in front of several of his soldiers and officials.

Austrian Chancellor Schuschnigg, not too fond of the NS party, had plans to do his own referendum on the 13^th of March but, informed of this by Hitler's spy the Home Secretary Arthur Seyß-Inquart, German troops cross the border into Austria unopposed on the 11^th and a day before Schuschnigg's referendum would've taken place the Führer announces the Anschluss.

On the other hand there is no denying that a great many Austrians turned up for Hitler's speech on the Heldenplatz in Vienna.

The fact is that after the mighty Austrio-Hungarian Empire had crumbled away in the wake of the first World War and just been left with the little core of German Austria many Austrians thought that the new country couldn't survive on its own, surrounded by many bigger nations they had either been at war with or that had just split off on unfavourable terms. So the new Austrian government originally intended to unite with Germany but that was quickly specifically forbidden by the Entente powers so as to not make Germany too powerful again.