From the FAQ on the bounty system (emphasis mine):
Twenty-four hours after the end of the bounty period, if the bounty starter has not manually awarded the bounty, an eligible answer can be automatically awarded half the bounty amount. The criteria for an answer to be eligible for automatic awarding are as follows:
- The answer must be given after the bounty was started
- The answer must have a score of at least +2
Fine by me, but why exactly? The reasoning behind it is not explained in the FAQ, it's just stated as fact.
As for the main reason for the bounty system, according to the FAQ it's this (emphasis mine):
This feature was designed to motivate answerers, and help questions get the answers they deserve.
If and when the bounty is awarded automatically, there is no motivation for the original answerers (to improve their answer) since in that case they are 100% certain they will not be awarded the bounty. Of course, answerers don't know up-front if this will happen, so this point may be irrelevant.
Now deserved seems subjective to me. If the total number of votes is a deciding factor when the bounty is awarded automatically, then doesn't that mean that the most deserving answer is decided on by the community? In that case, why would original answers (given before the bounty period) with a even more votes not be 'deserving' enough?