DHCP Lease Time Expiry is much longer than configured

Hi, All,

Using a Fortigate 100D here and configured it as DHCP server to a VLAN, the odd ball is, in the DHCP server setting DHCP Option 51 is set to 43200, however, on the DHCP Monitor, the client shows with expiry time at 19 Days +, do you see the same as well? Is there something i have missed. Appreciate any advice!