Bounce Rate Inconsistencies



I've been running a simple pages report and applying the default bounce rate metric (Bounces / Entries) and for a given date range of 4/1/18 to 4/15/18 I got a 30% bounce rate when I ran the report on 4/17/18, however, the bounce rates keep changing when I ran the same dates 1, 2 even 3 weeks later. Can someone explain how a bounce rate can change 2-4% over the course of a month depending on when I run it yet I'm running the same date range.