First, why I'm not surprised to learn Louise's dress was a repeat? Edward and Sophie clearly don't have funds to afford a new dress for their daughter once a year.
Palma Easter is a different season (spring), different locale (island casual), different age (Leonor was 9). Plus, the Infantitas didn't wear "a bunch" of short dresses. It's only Leonor who wore a shorter skirt last year and there were comments about the length of the skirt at that time as well. In all other years, the infantitas wore knee-length dresses or wore pants as this year. If we want to compare, we should look at their coats/dresses at the National Day. The Infantitas wore new dresses/coats every year. The coats they wore this year were knee-length and fit them just right. That's the occasion where people were telling Sophie to take note.
Spanish Infantitas seldom repeated any outfits in those few photo sessions through the years. They were almost always impeccably dressed and groomed from head to toe. In most of their summer photo shoots, Letizia would appear in very casual clothes but she made sure her girls dressed impeccably. It's the opposite of Sophie.