Classifier-Free Guidance Formulation

In the classifier-free guidance paper, the formulation is as follow

However, it is implemented in DALL-2 as null_logits + (logits - null_logits) * cond_scale According to the formula it should be logits + (logits - null_logits) * cond_scale where the logits are the conditional score and null_logits are the unconditional ones.

Can you please clarify the reason for this modification? Thank you!