In Windows 2000, leases that expire, but fall within the 4 hour grace period, are considered "in use", until they are scavenged from the database.
If you view these leases in the MMC, they appear as Expired, but they are counted with the active and excluded leases as "in use". You may even notice the Scope Full icon, because this display is based upon "in use" addresses, even though these grace period address are available to be distributed to clients.
Sample event ID 1020:
Event Type: Warning Event Source: DhcpServer Event Category: None Event ID: 1020 Date: 5/11/2000 Time: 5:32:24 PM User: N/A Computer: WIN2000 Description: Scope, 10.1.1.0, is 100 percent full with only 0 IP addresses remaining.