I have eaten the plums that were in the icebox (1-4)
Maybe he knew someone was saving the plums before he decided to eat them, maybe he only realized that afterwards, but either way, our speaker ate them. We don't get much deliberating about this decision—instead, we only see the speaker trying to decide how to handle his mistake. And we see here that he handles it in a very straightforward way—this is a confession any detective would love, no DNA tests needed here. Whatever the case, at least he's copping to his crime.
you were probably saving for breakfast (6-8)
These lines show even more of the courageousness of this apology—he's admitting that he knew someone else was saving the plums, and that he ate them anyway. Hey, at least he's honest. So now our imaginary detective not only has a confession, he's got proof that the suspect knew he was doing something wrong. Looks like this will be an easy case to win.
Forgive me (9)
Now this is an unusual, admirable move for a guilty person. Anyone who has struggled with their own pride after doing something wrong knows how tempting it is to always blame someone or something else. The detective might start getting a little uneasy now. The jury might not be able to refuse such a straightforward plea.
they were delicious so sweet and so cold (9-12)
These lines are weird for an apology, and, in their own way, kind of hilarious. We can imagine the speaker grinning, thinking about how yummy the plums were, and then writing about them, forgetting (or maybe not) that the person he's writing the note to won't get to eat the plums in the first place. But, instead of having a malicious intent, we think our speaker was trying to share some of his satisfaction, and give a little deliciousness to the person reading the note. At least that's a happy ending.