Can any body tell me why FLUENT is using default pressure and momentum URFs as 0.3 and 0.7. Based on what FLUENT decided these default values for URF's

Have a look at section 25.9.2 of the user manual. Here is an extract:

In FLUENT, the default under-relaxation parameters for all variables are set to values that are near optimal for the largest possible number of cases. These values are suitable for many problems, but for some particularly nonlinear problems (e.g., some turbulent flows or high-Rayleigh-number natural-convection problems) it is prudent to reduce the under-relaxation factors initially.

I never use the default, get faster convergence and lower residuals level when others. What is the logic on using 0.3 for pressure when using simple or simpler