I will do as I usually do with these kinds of debates and cite some historical anecdotes from the world of armored warfare.
During the Battle of the Korsun–Cherkasy Pocket in 1944 a company of Soviet T-34/85's surprised a group of five German Panzer IV's that were parked on a line while their crew was maintaining them. The command T-34 fired twice, hitting two PzIV at the same place in the mid hull behind the tracks where it would penetrate the ammunition lockers and cause a devastating explosion. The first tank blew up just like so, but when he fired the second shot, just a few seconds afterwards, it didn't penetrate the second tank. It got the commander thinking once they were done shooting the Germans to pieces, why? It had been the exact same distance, the exact same ammunition, shot in an almost indistinguishable angle at the same sort of tanks. Yet, one bounced while the other didn't. Why?
Truth is, the results can vary from shot to shot depending on a multitude of factors. In the above example, we have already eliminated differing positions, angles, types of target and other variables such as cover and if the target has already been shot at. However, there are hidden variables. It might have been that there was a flaw in the steel composition of the first tank (a common problem with late war German tanks since their resources were dwindling). The second shot itself might have been flawed in a certain way, or the shell had just a tiny bit less propellant than the first shot. Once you come down to it, it might just have been pure dumb luck.
In the same way one can never say that two identical 'Mechs can take the same identical fire. Even in such seemingly precise things as energy weapons all you need is a little difference in the output of one circuit and the variable has been drawn to one way or another. We must not (and probably cannot even if we wanted to) sacrifice storytelling to mathematics. Consistency is one thing, counting armor down to the points and comparing it to the damage of a weapon a whole other.
In the end we are role-playing here, interacting with characters we have created that pilot 'Mechs that we want with availability (no matter how rare) as our only guideline. We are not playing a tabletop game. We are not professional writers either (or at least I don't think so). One ton of armor and what it protects against is not that important to me at least and I won't hold it against any of you if you shrug off a PPC bolt in your Heavy or dodge a gauss round with your Light. As long as it is within the plausible realm and serves a purpose in the scene, I am as happy as could be. Everything else is what we have edit-buttons and OOC-threads for. In that regard I agree fully with Kayla's insider info: As long as it looks about right and suits the story, then it's what we go with.