The jet properties of Mrk 501 and 3C 279 are derived by fitting broadband spectral energy distributions (SEDs) with lepton models. The derived gamma(b) (the break Lorenz factor of the electron distribution) is 10(4)-10(6) for Mrk 501 and 200 similar to 600 for 3C 279. The magnetic field strength (B) of Mrk 501 is usually one order of magnitude lower than that of 3C 279, but their Doppler factors (delta) are comparable. A spectral variation feature where the peak luminosity is correlated with the peak frequency, which is opposite from the blazar sequence, is observed in the two sources. We find that (1) the peak luminosities of the two bumps in the SEDs for Mrk 501 depend on gamma(b) in both the observer and co-moving frames, but they are not correlated with B and delta and (2) the luminosity variation of 3C 279 is dominated by the external Compton (EC) peak and its peak luminosity is correlated with gamma(b) and d, but anti-correlated with B. These results suggest that gamma(b) may govern the spectral variation of Mrk 501 and delta and B would be responsible for the spectral variation of 3C 279. The narrow distribution of gamma b and the correlation of gamma b and B in 3C 279 would be due to the cooling from the EC process and the strong magnetic field. Based on our brief discussion, we propose that this spectral variation feature may originate from the instability of the corona but not from the variation of the accretion rate as does the blazar sequence.