The reason is the so called transaction charge. If you bet manually you never pay it. But if you let BA adjust your odds busily on many different markets then you may have so many transactions that you have to pay it. The "nice" thing about it is that you don't see it in "Betting P&L" at all and in your "Account Statement" it only comes up as "debit". So for the heavy users of BA it might be a good idea to scroll down their "Account Statement" if there is somewhere a "debit" instead of a betting event.
Of course it would be wounderful if there was something like a transaction counter in BA to avoid such problems in the future. But I am aware of the fact that other improvements of BA might have priority.
probably been charged a few times but never noticed it before as like Castillo said you don't get any notification and I doubt many of us go through our account statements when placing lots of bets.