Dispersion, bbs

How is the dispersion bbs data calculated? I did a verification on a small sample of 2 hands and the calculations don't add up. In one hand the pot is 19bb, but hero wins 17.7, therefore paying 1.3bb in rake. In the second hand the pot is 41, but hero wins 38.3, therefore paying 2.7bb in rake. The sum of rake paid should therefore be 4bb, and that's the data I expected to see under dispersion bbs, but the data that appears in the software is 4.77