I've set up an ISC DHCP server (Debian 7), and used a copy/paste version of a working configuration file. The DHCP server is multihomed in 3 physical networks.
There are dynamic IP ranges configured for each physical subnet, and fixed-address entries for static hosts.
Now, some hosts got the correct static fixed-address by MAC address, and other static-configured hosts were always IPs from the dynamic pool.
Removed leases from "/var/lib/dhcp/dhcpd.leases" and restarted isc-dhcp-server several times: No effect.
Double-checked name resolution, MAC typos, DNS name typos, etc: No effect.
[SOLUTION]
I was working in the backup of the config file. Therefore, nothing I've tried actually had any effect.
So, next time things just seem to "not make any sense": Make sure you're actually working in the right configuration file
(How embarrassing...
)