This is an understandable mistake, because if someone hears about a trial that reminds them of what they went through, it's naturally going to dredge up old feelings. And then they'll try to use those resurfaced emotions to show empathy, like a cat regurgitating a hairball made of love. But if the person they're talking to mourns differently, they've put them in the awkward position of either agreeing to something that isn't true or explaining, "Well, I do miss my grandmother-in-law dearly, but I actually feel no desire to go into a Home Depot and burn down the chainsaw department like you did."
"Really? I'm surprised, because I couldn't even hear a chainsaw for months without tracking it down and throwing it into the nearest lake." And suddenly, there's the implication that there's a proper way to be sad. If you aren't following the prescribed plan, you're not doing it right.
"You call that sobbing? My grandmother's sobbing harder, and she's dead! Because, you know ..."
The last thing a person needs to hear is that there's something wrong with how they work through their pain. A better approach is to simply say that you went through a situation like theirs, without going into detail. That gives them the option to talk about their feelings if they want to, and acting as a shoulder on which they can cry, vent, or plan vengeance is the most valuable thing you can do in a time of mourning.