L6203 H bridge (very) high Rds on
I have a feeling that Rds-on here is (per leg) so you need to multiply it by 2 to compare it with delta(V)/I.
Many integrated H-bridge IC specify the Rds-on for the high side FET and for the low side FET. Take for example this datasheet. 

Returning to the definition for Rds-on, it is easy to see why and to expect this.