This happens because you have the "Adjust for commission" checkbox ticked. When this is ticked, TheBetEngine automatically increases the stake to cater for the commission that would have to be paid if the bet were to win.
Taking a £5 example, and assuming a commission rate of 5%:
If I placed a lay bet at a stake of £5 and the bet was successful, then I would expect to receive a profit of £4.75.
However, if "Adjust for commission" is ticked, then the bet would be placed at £5.26, which would yield of profit of £5.00 if the bet were successful.
When calculating the increased stake, TheBetEngine looks at both the commission that is payable on that particular market and the current commission discount to which you are entitled.