Regardless of how, why or frequency, don't let it bug you.
My very first game of D&D- waaaaay back in the library of East Middle School in Aurora, Colorado in 1977- concluded with my human fighter and another guy's human Mage (down to his last MM spell) facing off against a Purple Worm.
We tried to back out of the room, but it was hungry and we were on the menu.
So the Mage launched his MM while my guy positioned himself between his ally and the worm.
My dice went cold- the Mage actually was doing more damage with his darts and then his staff than my fighter's 2Hd sword. Finally, I got a couple of solid whacks in that really turned the tide of battle.
Then the Worm got a 20...and et the Mage.
A few more rounds go by with the fighter and Worm trading blows. Then came the round in which the 2 combatants had simultaneous initiative...and my guy was down to 4HP.
I rolled poorly, the DM rolled another 20- end of game.
I was bummed for 2/100000ths of a second, but the way things went was sooooooo cool that everyone in the room was stoked.
As we were cleaning up, the DM revealed that the Purple Worm had been whittled down to 4HP as well. It didn't matter- I was hooked and I knew it.