The difference is that early-bird pricing is transparent and predictable. There is a written, known policy of $X discount during specific hours. You can plan for it. It's never a surprise.

Dynamic pricing means sometimes you go there, and Wendy's decides on the fly whether you get a lower price and how much. It gives Wendy's the option to pinch pennies how they see fit for their own benefit, rather than offering a deal which you can choose to accept.