xanatos, the custom start function acts as a 'zoom-in' into your account's growth. Your way was also considered at the time of development, however we wanted to reflect the original account's growth so if someone 'zooms out' by selecting the full history of an account, the growth section in the period of the custom start will remain exactly the same.
If we were to do it as you've suggested, the growth line will not be similar to the original growth since the base deposit on which the calculations rely, will be completely different.
As to the differences between cumulative gain and absolute one - the absolute one does not take into account interest, it's a simple (profit/deposits)*100 calculation.