Pull-up resistor I2C value; R(min) formula approximation
The output device (an N-channel MOSFET with open drain) is guaranteed to have no more than 0.4V across it when 3mA flows. 

In this region of operation (very low Vds), the MOSFET looks like a resistor of value Rds(on), and the guarantee on output voltage under load is essentially a maximum value for Rds(on)= 133.3 ohms in this case. If that resistance happens to be lower, say 100 ohms (which it usually will be, since all chips must meet the upper limit) then less than 0.4V will be dropped with a resistor that is chosen to give 3mA with 0.4V drop (and the current will be slightly higher- 3.062mA). 

This is a consequence of the MOSFET behaving linearly, just like a resistor.