The rays reflected by mirrors M1 and M2 meet at the glass G. For each ray(specifically 1 and 2 in the figure which may go on 1,2,3... for an extended object as is required for the experiment) as the splitted beams meet at G the path difference for each ray(1,2,3..) would be the same (x2-x1) according to the figure, then why do we see circular fringes at the screen?
It is similar to the interference pattern of a slits or particularly a round openings. The perspective is in line with the center of the rings but perpendicular to the surface of the mirrors. Consider the light beam from the source to the center of the pattern as straight. Then every ring out from there corresponds to a particular value of theta. These are fringes of equal inclination.