I see the same in a 'no ip routing' switch with a difference in the metric of the route:
sw01-12p(config)# ip default-gateway 192.168.3.1
sw01-12p(config)# show ip route
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
0.0.0.0/0 192.168.3.1 3 static 250 1
sw01-12p(config)# ip route 0.0.0.0 0.0.0.0 192.168.3.1
sw01-12p(config)# show ip route
Destination Gateway VLAN Type Sub-Type Metric Dist.
------------------ --------------- ---- --------- ---------- ---------- -----
0.0.0.0/0 192.168.3.1 3 static 1 1
As mentioned before, how I learned it is that in no-routing mode you will use ip default-gateway, and the switch will not do L3 routing for traffic. In ip routing mode you will use ip route. It seems to be that in routing mode the ip default-gateway will not be honored for routed traffic, just for the device traffic.