Originally the Chief Rabbinate of Israel ruled that one should say Hallel without a bracha on Yom Haatzmaut ("The Chief Rabbinate ruled to recite Hallel without a blessing." http://hirhurim.blogspot.co.il/2007/04/hallel-on-yom-ha-atzmaut.html?m=1; "Because of all, or some, of these concerns, the Chief Rabbinate’s Council originally prescribed that one recite the Hallel without a blessing during the morning prayers of Yom HaAtzmaut." ph.yhb.org.il/en/05-04-07) and with a bracha on Yom Yerushalayim ("The Chief Rabbinate, in a ruling signed by Chief Rabbis Isser Yehuda Unterman and Yitzchak Nissim, along with the renowned R. Shlomo Yosef Zevin and R. Shaul Yisraeli, ruled that Hallel on Yom Yerushalayim should be recited with a blessinng." http://etzion.org.il/en/celebrating-yom-ha-atzmaut-and-yom-yerushalayim) This is because Yom Yerushalayim is more clearly a miracle as a definite military victory occurred then, whereas Yom Haatzmaut was merely the day of a declaration. Perhaps some people extended this reasoning.
