the rates can be quite different between the exchanges you are mentioning, as well as others. Especially at the times of high volatility (which is effectively NOW and in the last several months).
At any given time, the rate we are getting may be worse than any of the sources you mention, hence the difference.

We are not charging 5% (or more, as some particularly smart and loud voices are stating here).

No one seems to be shouting about the much better than market buy bitcoin rate that we are offering, by the way. Why could that be? Always easier to be negative, isn't it?