Mr. Radley tells Jem that he filled the hole in the tree with cement because the tree was dying, and plugging the holes with cement was what you did when a tree was dying.
We can infer from the story up to this point that there are two reasons why Jem might be skeptical of this explanation. The first reason is that he and Scout have become accustomed to finding gifts in the hole, like coins, a couple of figures carved from soap, and a pack of gum. The second reason is that Mr. Radley has a reputation amongst the children as something of an ogre. Indeed, in Chapter 1, Jem is said to believe that "Mr. Radley kept (his son) chained to the bed most of the time." From Jem's perspective then, it seems logical that the ogre-like Mr. Radley would block the hole in the tree simply to be unkind and put an end to the succession of gifts.
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