Two-dimensional (2D) electron gas at interface anodic oxide - HgTe (110) is studied experimentally (by magneto-capacitance spectroscopy method) and theoretically at carriers surface density up to 6 × 10¹² cm^-2. The measurements show the population up to four subbands with well-resolved Rashba spin splitting in Fourier transforms. The carriers distribution among electric subbands agrees with the theory. However, the experimental relative differences of occupancies of spin sub-subband (0.17-0.3) exceed the calculated ones (0.14). This discrepancy testifies to the interface contribution to spin-orbit splitting. The partial capacitance oscillations for different spin branches in ground subband differ not only by the period but by the amplitudes also. Because of this the measured effective cyclotron masses in this subband correspond to the theoretical values for high-energy spin branch whereas in excited subbands to an average over two branches.