A common cause is maybe the fact some users have configured static ip address on their machines.
Depend on your network usage and working hours, you could try to shutdown the dchp server before all the machines join on the network and discover any IP still enabled on the network. This tell you that those machines have a valid IP address not provided by the dhcp server. But I’m not sure if this applies to your environment according to your posibilities.
Hope it helps.
Could be a couple of causes:
1. Clients are not using DHCP but are using static addresses.
2. Clients not releasing leased addresses properly. – check lease times and client patch levels.
3. Rogue DHCP server on LAN. – check where clients get bad addresses from