Boiling eggs and other delicacies

In our house, we share the cooking duties.
I make most of the lunch and evening meals and Marc cooks the eggs.
(He also mans the barbecue and the wok as well but this column is about eggs.)
He likes a hot breakfast with choices.
So since I was usually leaving before he was in the mornings, over the years we developed a routine.
He cooked breakfast and I ate it.
I always thought it was pretty much a convenience thing.
But the other day after I left him to make a potato salad with eggs I had boiled the day before, I discovered something.
I apparently can't boil eggs.
I had boiled four extra eggs for him thinking I was saving him some time and effort for the salad he was making to take to a work party.
Off I went to shop.
The next morning I was going about my business when I noticed eight eggshells in the garbage not four.
"Didn't you use the eggs I left you?" I asked him.
He kind of hemmed and hawed not wanting to make me mad.
"They were too soft," he finally admitted. "I made new ones."
I was confused. I had made them the same way I always did.
I put them in water, brought it to a boil and cooked them for five minutes...with a timer to remind me when time was up.
"Isn't that how you do them?" I quizzed in a righteous huff.
Marc looked at me sadly.
"They need 15 minutes minimum," he said, ready to duck my blows.
"Uh uh! I'm sure it's five. I know how to boil eggs."
I headed to the cookbook and sure enough. They suggested 15 minutes for hard-boiled eggs. Five for soft-boiled.
For more than 40 years I've been doing it wrong and wondering why sometimes my eggs turned out runny when I needed them hard-boiled.
I always thought it might be the altitude.
Now I see it was more accurately my attitude.