The units of pressure and velocity are not the same so there is no expectation

that the tolerances would be the same.

The velocity generally converges much more rapidly so there is no cost in making

that tolerance tighter and doing so generally helps to keep things relatively smooth.

Thus, the key is - what tolerance for the pressure.  In non dimensional units based

on convective timescales 1.e-6 is generally found to be adequate for PnPn-2; a bit

looser is ok for PnPn.

In some cases (e.g., if you are measuring very small lift values for flow past a cylinder,

say) you might need tighter tolerances, but these work for most applications.

For transition or for "exact" solution comparisons, you would also want tighter 

tolerances, e.g., 1.e-10/1.e-12 or even 1.e-12/1.e-14.

You have to be careful to not get too close to machine precision because the condition

numbers of the governing system matrices will prevent you from converging to 

tolerances that are too tight.

hth,

Paul