That's a good improvement, but not a considerable adjustment compared to your average that I would classify as unusual. If this was a 15% jump, then I would look into what you have been doing and to keep going with it. Realistically I imagine it must have had to do with who and what you were sending since May 3rd.
Perhaps the explanation is:
- you emailed more clients this past week, thus these email addresses are more verified and more likely to deliver and will increase your overall delivery rate
- you emailed less prospects this past week, this these email addresses are less verified and less likely to deliver and will decrease your overall delivery rate