Why Sage Markandeya did not know that Rama performed 100 horse sacrifices?

Book 3 - Book of Forests - Aranya Parva Chapter 275 - aadhyaaya 275 Verse 069 - slooka 069
tatoo deevarshisahitah saritam gomati`m anu das`a`s`vamedha`n a`jahre ja`ru`thya`n sa nirargala`n.
Yudhisht`hira and his brothers were in the forest, on an exile of twelve years. Sage Markandeya was consoling him by narrating stories. Markandeya wanted to impress on Yudhisht`hira that he was not the only person who suffered in the forest. Markandeya narrated the story of Rama who suffered without any accompanying supporters, except his brother Lakshmana. This verse occurs after Rama's coronation. Gist: Rama fluently performed on the river Gomati, ten horse sacrfices with the help of Gods and Sages.

BLOGGER'S OBSERVATIONS
Rama performed 100 horse sacrifices according to Valmiki Ramayana.

It is not clear why Markandeya reduced the number by 90 percentage points.

Vyasa Mahabharata also has references to the presence of Sage Valmiki along with other sages. Did Markandeya hear the Rama's story from Valmiki's mouth or through some other sources?

Is Valmiki also a chiranjivi (person who lives eternally without death) a la Markandeya. Then only Valmiki can be present both in Rama's court and Dhritarashtra's court.

Markandeya was an adviser to Sage Vasisht`ha in Ayodhya Kanda, of the Valmiki Ramayana.

Markandeya probably ought to have informed Yudhisht`hira that Rama ruled Ayodhya for 11000 years. This would have lifted the spirits of Yudhishtira who hasd been suffering in the forest!