If you've got a good grasp on queue theory, so you prepare for the talk by assuming an M/M/1 and working out the arrival rate of tickets vs. the rate you complete work at. You work out probability (uncompleted items >= tickets not done).

Now you're in a great position to negotiate workload because you have all the figures to work out what just happened - is the problem that you completed tickets too slowly, that there were too many tickets, that even basic variance in task completion rates would result in this happening 5% of the time, et cetera. You have on hand immediately how common this must be so you are in a position to guess at how you compare to colleagues. You can make relatively low-ego determinations about whether this is a sprint-specific mistake you made or if it was all but certain to happen due to basic task variance.

There are a lot of conclusions to be drawn there just by a cursory review of past performance. There isn't anything magic to it, but you're going to be able to spend the conversation worrying about the social aspects of how to manage said boss and don't have to waste valuable brain cyles working out what just happened yourself. You only need to remember 2 numbers and a few napkin-level formula. Works even better if you happen to be the boss because now you can make some quick guesses about whether there is a problem here or just statistical variance.