Modern books churn: a large share of posted orders are cancelled within milliseconds. Some of that is legitimate risk management, some is queue positioning under pro-rata-allocation, and some is deliberately misleading.
The practical lesson is to discount displayed depth and judge liquidity by what actually trades. Volume at a price is evidence; size at a price is a claim.
Example: a level shows 12,000 shares bid. A 5,000-share market sell fills 1,100 at that price and the rest two ticks lower, and the level then redisplays 11,000. The book showed 12,000, the market absorbed 1,100, and the other 10,900 were never available to you.
Related: quote-fade, order-book-imbalance, depth-of-market, spoofing