Breakdown of Le petit-déjeuner précède le déjeuner.
Questions & Answers about Le petit-déjeuner précède le déjeuner.
Because they are related words. In modern standard French, petit-déjeuner means breakfast and déjeuner means lunch.
Historically, déjeuner was connected with breaking the fast, and petit-déjeuner literally means something like small breakfast/early meal. Over time, the meanings settled into the modern meal names used in France.
French often uses the definite article where English does not. So le petit-déjeuner and le déjeuner can mean breakfast and lunch in a general sense, not necessarily the breakfast and the lunch of one specific day.
That is very normal in French: