The Curiosity and Perseverance Mars rovers include the laser-induced breakdown spectroscopy (LIBS) instruments ChemCam and SuperCam, respectively, to assess the chemistry of Mars surface materials. Onboard calibration targets (CT) are frequently analyzed to check for system health. Their spectra and predicted compositions are uploaded to the NASA Planetary Data System (PDS). The ten ChemCam CTs cover limited compositions, while the 23 SuperCam CTs are more representative of Mars. This study compares and contextualizes predicted versus actual abundances of major elements (SiO2, TiO2, Al2O3, FeOT, MgO, CaO, Na2O, and K2O) in the CTs. For both instruments, Mars-based calibration accuracies differ from PDS-reported values: Curiosity on-Mars errors are equivalent or better than lab values but from CTs with limited scope. Perseverance has poorer Mars-based accuracies than expected in the lab, but CTs are diverse. Results over time reveal that CT surfaces have not degraded and Mars-based calibration accuracies are mostly reproducible.