This difference is related to the difference of Pharmacokinetics between Dexamethasone and predinsolone ,as Dexamethasone has the longer half life.

Because dexamethasone has a long half-life, Altamimi et al attempted to determine if a single dose of dexamethasone (0.6 mg/kg) was equal to 5 days of prednisolone (2 mg/kg daily). The single dose of dexamethasone demonstrated no difference in any of the following: hospital admission rates (13.4% dexamethasone vs 14.9% prednisolone), additional β-agonist therapy, return to baseline of patient self-assessment scores (5.21 days vs 5.22 days, respectively, mean difference −0.01; 95% CI −0.70 to 0.68), and mean pulmonary index scores (0.4 vs 0.3, mean difference 0.1; 95% CI −0.25 to 0.45) in children 2 to 16 years of age with mild to moderate asthma.

Reference:
Use of dexamethasone and prednisone in acute asthma exacerbations in pediatric patients
PMCID: PMC2718595
PMID: 19602654