If you look at your ecash statement (found under the Account & Preferences/My Account tab), it shows a complete breakdown of all transactions (which you can print out for study offline, if you like).
The total credits/debits numbers are the totals of the two columns on that statement. Basically, one is the amount you sat down at a table with, and the other is the amount returned to your account when you got up again.
In your example, you took $100 from your account to buy chips at a table, and then you converted those same chips back into $100 again, so the total of each type of transaction rises by $100.