It's because they're both fucking useless and you should ignore (and remember to always ignore in the future) anyone dishonest enough to pretend that you can draw conclusions about voters under 30 from either one of them.
The NY Times/Sienna poll had 179 18-29 year old respondents while the Economist/YouGov poll had 168 18-29 year old respondents; those sample sizes are of zero scientific value.
If you're wondering how this poll could so completely disagree with this poll:
It's because they're both fucking useless and you should ignore (and remember to always ignore in the future) anyone dishonest enough to pretend that you can draw conclusions about voters under 30 from either one of them.
The NY Times/Sienna poll had 179 18-29 year old respondents while the Economist/YouGov poll had 168 18-29 year old respondents; those sample sizes are of zero scientific value.