If you analyze the same data with a t test and the nonparametric Mann-Whitney test, which do you expect to have the lower P value?
If you assume that the data are sampled from Gaussian distributions, then the t test has a bit more power (depending on sample size) so will -- on average -- have a lower P value. But only on average. For any particular set of data, the t test may give a higher or a lower P value.
If you don't assume the data are sampled from Gaussian distributions, then the Mann-Whitney test may have more power (depending on how far the distribution is from Gaussian). If so, you'd expect the Mann-Whitney test to have the lower P value on average, but the results are not predictable for any particular set of data.
What does "on average" mean? Perform both tests on many sets of (simulated) data. Compute the average P value from the t test, and also the average P value from the Mann-Whitney test. Now compare the two averages.