Why is this an issue?

While most compareTo methods return -1, 0, or 1, some do not, and testing the result of a compareTo against a specific value other than 0 could result in false negatives.

Noncompliant code example

if (myClass.compareTo(arg) == -1) {  // Noncompliant
  // ...
}

Compliant solution

if (myClass.compareTo(arg) < 0) {
  // ...
}