The substantial majority of the @Harvard endowment is invested in illiquid assets, principally private equity, real estate, and venture capital. Not reflected on the balance sheet are commitments to new funds of the same type.
Like most endowments, @Harvard models expectations of fund distributions when considering its liquidity and when making future commitments.
Harvard also makes assumptions about inflows from alumni donations.
The model likely did not predict a decline in liquidity events from private equity, real estate, and venture capital and the dramatic decline in donations. That is likely why Harvard announced this recent bond offering, which is being done in a substantially higher interest rate environment than where the funds could have been raised a couple of years ago.