The chain where the blocks can be computed using less work bought that privilege by reaching the retargeting later, so there is not much to gain.
Well, whether or not there is much to gain depends on how long the fork lasts for.
Or are you talking about a timestamp manipulation attack?
Also a possibility, as I've discussed higher on in this thread:
https://bitcointalk.org/index.php?topic=5452676.msg62269397#msg62269397