The rows of circuit breakers in your panel alternate between connecting to the left phase and the right phase of the split-phase supply. Your IoTaWatt is using a single voltage reference, connected to just one of the phases, to calculate power for all of the breakers. So all of the breakers connected to one of the phases will be positive, and all of the breakers connected to the other will be negative because they are using the voltage reference from the opposite phase, which is the exact opposite voltage.
To better understand this concept of the voltage of one phase the opposite of the other, consider that your panel supplies both 120V and 240V. Notice that wherever you have a 240V breaker, it’s actually two breakers in adjacent rows of the panel, so the two breakers are on opposite phases. When one phase alternates to +120V, the other alternates to -120V. The difference between the two is 240V. You can measure the total power in that circuit by passing the two 120V conductors through the same CT, but you have to reverse one of them through the CT because that reversed conductor is connected to the other phase, where the voltage is reversed.
While the 120V circuits will automatically correct if reversed, you need to understand this concept to properly measure 240V three-wire circuits. That is explained in the referenced docs.