The "odd" factor relates to their age. In Chapter 3, we are told that Mae did not look in the mirror because her reflection has not changed in 87 years. In Chapter 5, when Winnie meets Jesse, he tells her that he is 104 years old. However, Winnie doesn't believe him because he only looks 17. The Tucks do not look their age, and that is what is weird. As the book continues, of course, we learn that the reason they don't look their age is because of the water that they drank to make them immortal. The details in Chapter 3 and 5 specifically are an example of foreshadowing. Babbitt is letting the audience know that something is odd and hinting at what that "oddness" is before giving the full story to the readers. This builds suspense and expectation.
What is also interesting about the way the Tucks look is that they have been preserved at relatively young ages, even the adults. However, while this might seem to be a blessing, it is really a curse - the Tucks, like Mae in Chapter 3, are bored with their never-changing reflections and would like to be able to see themselves grow and change.