It looks like you're using a visit-based segment, which will impact both metrics you're comparing.
- Visits that start late on Feb 28 2017, match your segment, then bleed into March 1 2017 will be included in your first column, but not your second column.
- Visits that start late on March 31 2017, bleed over to April 1 2017, then match your segment will be included in your second column but not your first.
Since we're talking about something like a 0.0003% variance, It probably isn't something you need to worry about.