So, this is true in two (not really independent) senses I can think of. First, in most cases, there isn’t enough money chasing shares to sell all shares at the current price. Second, the act of shareholders selling in large numbers is new information that itself changes the price. The current price is a marginal price, and we don’t usually know how steeply the rest of the price curve slopes at larger volumes for either buying or selling.
So, this is true in two (not really independent) senses I can think of. First, in most cases, there isn’t enough money chasing shares to sell all shares at the current price. Second, the act of shareholders selling in large numbers is new information that itself changes the price. The current price is a marginal price, and we don’t usually know how steeply the rest of the price curve slopes at larger volumes for either buying or selling.