That is because 'c' comes before 'd' in the alphabet. If two words begin with the same first letter, we then look at the second letter to decide on the alphabetical order.

The order of the alphabet became fixed soon after the letters were ... has found evidence of alphabetical ordering in Greek administrative texts as early as the second century BCE.

Thus all words having ‘a’ as their first letter were at the outset collected ... for Everything – The Curious History of Alphabetical Order (Picador, 2020). Sorting in general is the book ...