Bookings capture commercial activity in the period it happens. Revenue for durable virtual items is typically recognised over an estimated paying-user lifetime, so a purchase today can become revenue spread over many months.
That makes bookings the better read on current player spending, and revenue a lagged signal. The difference between them is largely the change in deferred revenue. In a growing business bookings run ahead of revenue; in a declining one, old deferred revenue can mask a drop in current spending.