dhcp ddns not working for fixed-address?

I have one subnet containing one host with a fixed address and two address
pools for dynamic assignment (see my dhcpd.conf below). Everything works,
the clients get their IPs, DHCP registers the IPs with DNS, the fixed
address host gets the address I assigned to it. But DHCP doesn't register
the IP for that host, but only for the clients that get dynamically chosen
IPs. Is this default behaviour? Can I change this? Is anything wrong with my
setup?

Regards,
Thomas

This is my dhcpd.conf:

ddns-update-style interim;
ddns-domainname <domain name>;
ignore client-updates;
update-optimization false;

option domain-name <domain name>;
option domain-name-servers <dns ip>;
option ntp-servers <ntp ip>;

# one week
default-lease-time 604800;
# four weeks
max-lease-time 2419200;

authoritative;

class "servers" {
match if <condition>;
}

subnet 10.10.0.0 netmask 255.255.0.0 {
option subnet-mask 255.255.0.0;
option routers <router ip>;

# servers with fixed ip addresses
host <fixed ip host> {
hardware ethernet <mac address>;
fixed-address 10.10.5.100;
}

# dynamic server pool
pool {
range 10.10.5.200 10.10.5.240;
allow members of "servers";
}

# workstation pool
pool {
range 10.10.100.100 10.10.100.199;
deny members of "servers";
}
}

key dhcp {
algorithm HMAC-MD5;
secret <secret>;
};

zone MINDMATTERS. {
primary <dns ip>;
key dhcp;
}

zone 100.10.10.in-addr.arpa. {
primary <dns ip>;
key dhcp;
}

zone 5.10.10.in-addr.arpa. {
primary <dns ip>;
key dhcp;
}