Rds(on) calculation in MOSFET datasheet
Look at figure 9 - I've copied it and extended it just to show that both on-resistance numbers are consistent: -



Normally fig 9 goes up to a drain voltage of 2 volts but if you project the lines you will see that at 5V drain voltage the current is about 300 amps and therefore on-resistance is about 17 mohms.

At 80 amps the on resistance is about 3 mohms. Figure 10 is like figure 9 but at 175 degC.