Thus Sri Rama is related to Krishna as Krishna's Great Aunt's descendant ( removed by Thirty Seven generations.
This means the information about Mandhata and Muchukunda was deliberate and people knew there is this discrepancy of longevity.
Mandhata. He was the Seventeenth King in the Lineage of Solar Dynasty. Sri Rama was the Fifty first in that Line.Mandhatha and Sri Rama were separated by eighteen Kings