Motor skills. The common developmental charts are dominated by motor skills early. Since these are based on nervous system maturity, it is only fair to a premi to base expectations on their conceptual age rather than time of birth. This means we adjust expectations by however weeks they were early. After 18 mo or so they are generally doing everything the same.
Multiple factors. A premature infant's development is assessed by corrected age. 40 weeks minus the gestational age at birth yields the number of weeks to subtract from an infant's chronological age at the time of the assessment till age 2. An 8-month-old infant born 12 weeks early is tested as if he were 5 months old. It's hard for some parents to let their nicu grads develop pro-social and coping skills.