In any class of five-year-olds, some children are better prepared by nature and/or nurture to learn to read, but the way early literacy is typically taught at present tends to exacerbate these differences and allow the weaker children to fail.
These children need more systematic, explicit teaching of sounds and spellings, or they can't get off the literacy launching pad. The good news is that this kind of teaching improves everyone's reading, and especially their spelling.
With smaller skill gaps between the children in their classes, teachers don't need to work so hard on curriculum differentiation.