Why ip route add doesn’t work, but ip route add with less details and then change works

I have a problem with ip route add. Surprisingly, I can add the route then change it, but I cannot directly add it:

# ip route add 192.168.0.0/16 via 192.168.255.254 src 192.168.1.101
RTNETLINK answers: No such process

but:

# ip route add 192.168.0.0/16 dev eth0
# ip route change 192.168.0.0/16 via 192.168.255.254 src 192.168.1.101

My interfaces file is:

iface lo inet loopback
auto eth0
iface eth0 inet static
    address 178.xxx.xxx.xxx
    netmask 255.255.255.192
    network 178.xxx.xxx.xxx
    broadcast 178.xxx.xxx.xxx
    gateway 178.xxx.xxx.xxx
auto eth0:1
iface eth0:1 inet static
    address 192.168.1.101
    netmask 255.255.0.0

I am a bit lost I must say. I am fine with adding then changing but it is not satisfying to not understand why it works in two steps and not directly. I have been reading man pages after man pages without understanding.

Updated with the help of the answers:

I have now a gateway on eth0 and eth0:0, but as I want all my traffic except the 192.168.0.0/16 to be marked as coming from my 178.xxx.xxx.xxx address, I added a routing rule. Maybe having eth0 with the 192.168.1.101 ip and then the 178.xxx.xxx.xxx ip on eth0:0 would work without routing rule, but here it goes:

auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
    address 178.xxx.xxx.131
    netmask 255.255.255.192
    network 178.xxx.xxx.128
    broadcast 178.xxx.xxx.191
    gateway 178.xxx.xxx.190
    up /sbin/ip route add default via 178.xxx.xxx.190 dev eth0 table 125
    up /sbin/ip rule add from 178.xxx.xxx.128/26 table 125
    post-down /sbin/ip route del default via 178.xxx.xxx.190 dev eth0 table 125
    post-down /sbin/ip rule del from 178.xxx.xxx.128/26 table 125

auto eth0:0
iface eth0:0 inet static
    address 192.168.1.101
    netmask 255.255.0.0
    gateway 192.168.255.254

I have also disabled rp_filter:

echo 0 > /proc/sys/net/ipv4/conf/eth0/rp_filter

Thanks for your help!