How to calculate entropy for IWT image decomposition ?

Hi, I wanna ask. If I already perform Integer wavelet transform and I did 9 level image decomposition (I use image size 512X512). How can I calculate the entropy in each subband? what I did right now is I just use entropy function from Matlab to calculate each subband entropy but the value seems wrong, it's too small. original entropy is 7.4455, entropy for the first level is 2.7753 and for the second level is 3.3481. the entropy value is not consistent. How can I verify the entropy is true?