It depends on the external markets and not on the internal markets. For example, SBD is traded lower on the internal markets because of less demand or trade volume. On Bittrex and hitbtc for example, it is traded at much higher volumes with more demand and so the price is higher there.

If you convert on the internal market, you will not get this same value for by your SBD.

This is why, although SBD has increased by much, the average value in USD on my account has not really changed.