Carbon dots (CDs) have attracted significant research interest due to their great potential in optoelectronic applications. Although various CDs have been synthesized via the "bottom-up" pathway, few studies have focused on understanding the origins of the structural and optical diversities of CDs. In this study, two benzenoid acids with a slight structural variation (i.e., 9-oxo-9H-fluorene-2,7-dicarboxylic acid (FR) and 4,4'-biphenyl dicarboxylic acid (BP)) are employed as precursors, yielding orange- and red-emissive CDs with quantum yields of 43.1% and 30.9%, respectively. A combined experimental and theoretical study reveals that the structural and optical diversities of the CDs originate from the structural variation of their precursors. Furthermore, we demonstrate that the light-emitting diodes (LEDs) based on the blended emissive layer of poly(N-vinyl carbazole) (PVK) and the synthesized CDs display cyan and yellow lights, respectively, with moderate turn-on voltages of 4.0 and 4.5 V and maximum luminance values of 454 and 276 cd m-2. Such different optoelectronic performances could be attributed to the different energy-level alignments of CDs-FR and CDs-BP, relative to that of PVK. This study thus provides a typical example to understand the precursor-dependent diversities of CDs, which may contribute to the rational screening of precursors towards CDs with desirable optical/optoelectronic properties.