Why does the speaker call the jam stain strange in the poem "Dear Mum"?

In the poem "Dear Mum" by English poet and author Brian Patten, the speaker is an absent boy who has written a note to his mother. Composed in comic verse, it is a catalog of all the household mishaps he is responsible for in her absence, but he does not take responsibility for the destruction. Instead, he professes ignorance as to how a cup was broken, a Ming vase cracked, a sink flooded, and the carpet tracked. He also denies knowledge of how the cat ended up in the washing machine or how his sister's rabbit ended up in the garbage disposal. The "strange jam-stain" is "about the size of a boy's hand" and obviously his own, though by calling it "strange" he distances himself from its origin. He ends by telling her that he thinks the house is haunted, so he is taking refuge at his grandmother's house.