Yes you are correct in your assessment that this should be based on a forecast which is all mapped out in clause 63.1. It stops the "cake and eat it" scenario for the client as you describe which is not what the contract says.
They may however use the actual to say the forecast was not realistic in first place (you said it would take 10 days and it actually took 2 days) but they still should allow what would have been reasonable as a forecast for risk. This same clause emphasises that if the actual cost was more than a reasonable forecast and the quote is not yet agreed you can not revert to actual cost either.
They may however use the actual to say the forecast was not realistic in first place (you said it would take 10 days and it actually took 2 days) but they still should allow what would have been reasonable as a forecast for risk. This same clause emphasises that if the actual cost was more than a reasonable forecast and the quote is not yet agreed you can not revert to actual cost either.