The DHCP server generates the list of available IP addresses 
from a hash table. This means that the addresses are not 
sorted in any particular order, and so it is not possible to predict 
the order in which the DHCP server will allocate IP addresses. 
Users of previous versions of the ISC DHCP server may have 
become accustomed to the DHCP server allocating IP 
addresses in ascending order, but this is no longer possible, 
and there is no way to configure this behavior with version 3 of 
the ISC DHCP server.

https://kb.isc.org/docs/isc-dhcp-44-manual-pages-dhcpdconf#dynamic-address-allocation