Is the difference of two positive rational number always positive explain

No . . . while the difference represents the absolute magnitude between two numbers . . . for example . . .

The difference between 5 and 2 is . . . 3
The difference between 6.4 and 9.5 is . . . 3.1

 . . . there is still the chance that the difference may be zero . . . in which case the difference is neither positive nor negative

. . . so in short . . . the answer is . . . NO