Gita 04.05 explained
bahūni me vyatītāni
janmāni tava cārjuna
tāny ahaṁ veda sarvāṇi
na tvaṁ vettha paran-tapa
Word for word:
śrī–bhagavān uvāca — the Personality of Godhead said; bahūni — many; me — of Mine; vyatītāni — have passed; janmāni — births; tava — of yours; ca — and also; arjuna — O Arjuna; tāni — those; aham — I; veda — do know; sarvāṇi — all; na — not; tvam — you; vettha — know; param–tapa — O subduer of the enemy.
The Personality of Godhead said: Many, many births both you and I have passed. I can remember all of them, but you cannot, O subduer of the enemy!
Although Shri Krishna and Arjuna were on same chariot but their levels are different
Supreme Lord never forgets things, we forget things, things are buried so deep in our mind and we forget things like in computer we forget the reference, file name and file location. We also forget things because of change of body as birth and death experiences are very dramatic. Unless we forget the painful things in life we will not be able to move forward. Even we don’t remember what we have done a few day back. For Bhagvan His body, mind and soul all are same.