Glow and Nuture say different

Erin • Christian, wife (5.17.14), Mom (Elliott- 6.7.18, Evan -June 23, 2021), pet parent, Fightin' Texas Aggie, radiological engineer

Glow says I am 4 weeks and 3 days but nuture says I am 4 weeks and 2 days. I can tell that glow is counting the first day of my last period but nuture doesn’t start counting till the next day. Why is that?