138 permanent deaths! Wow! How many sessions have you played?
Hmm... good questions. My 5e records also include D&D Next, so let's call it about 3.5 years of play. We play every Thursday night, for about 5 hours... and every other Saturday for about 7 hours. So, let's call it 17 hours every two weeks. If the "typical" D&D session is 4 hours, then that's the equivalent of 2 sessions per week. Multiply by 200 weeks, and we have approximately 400 sessions.
So, our rate-of-loss is approximately one character every three sessions. That sounds about right. Bear in mind that my group has had about a dozen TPKs, which means there are sessions where they'll lose 4-5 characters in a single go. That skews the numbers. It means we could go a dozen sessions without loss, and suddenly lose a bunch.
There are some things to keep in mind, which might explain differences with other groups...
1) I strictly control access to diamonds in our games, so
revivify and
raise dead are almost never available. If a character dies, they're usually dead. Diamonds, or scrolls of life-restoring spells, are treated as special rewards or can only be purchased at extremely high prices.
2) I control access to magic items, and there are no item-crafting rules or magic item shops. Many PCs won't have a magic weapon at 8th level, which makes damage resistances meaningful.
3) I roll all dice in front of the players, so no fudging to keep anyone alive.
4) I play the monsters appropriately according to their intelligence. I'll use ambush, hit-and-run attacks, summoned minions, mixed groups, disarms, grapples, target-the-casters,
silence spells, grab-and-fly-into-the-air, separate the party, and any other tactic I can think of other than just stand toe-to-toe and trade attacks. Particularly smart opponents will use flight, terrain obstacles (underwater, crawl-spaces, lava pools), magic (wall spells, passwall to open pits beneath characters, etc) or minions to split the party then tear them apart piecemeal. And, once a PC is down, the smarter monsters will usually keep them down. Take out the healers first, put at least one attack on any character that has just received a healing spell to restore their consciousness (usually, they're on low HP and prone, so can be taken out quickly again), and (if any PC is healed-from-unconscious twice in the same combat) coup de grace them the next time they go down.
So, yeah. We choose to make our D&D fairly challenging. And, also, my players tend to be extreme risk-takers. It's a shame when someone loses a character, but it's only really disappointing when they TPK. That almost always means the end of a campaign. Any non-TPK death is simply an opportunity to try out another concept, which can be a lot of fun.