The finite element methods, i.e., what COMSOL uses internally, does not solve the problem exactly. It only (i) provides an approximation to the solution, and (ii) does so in a way for which we can show that the approximation converges to the exact solution of the problem as mesh becomes finer and finer. This typically implies that on meshes that are too coarse the difference between the approximation and the exact solution will be large.
Convergence, however, also implies that as you make your mesh finer and finer, the difference will become smaller and smaller. In other words, as long as you see significant differences between the solution on one mesh and the next, you can expect that both are inaccurate and unreliable. On the other hand, if you see no significant differences on two or more finer and finer meshes, you may hope that the approximation is reasonably close to the exact solution and can be trusted to a certain degree.