Hi @Darren C
This is likely due to Klaviyo’s attribition window which is set to 5-days by default for emails (24hrs for SMS). More info here
So, even if your contacts open your email without clicking a link then purchase via another channel (social, direct, organic etc), Klaviyo will attribute those sales to your May campaign if the purchase occurs within the 5-day window.
Another consideration is that Klaviyo works on “last touch” for campaigns and flows. For example, if a contact receives and opens your May campaign then purchases by clicking on a link from another campaign email (or flow) that was received prior to the May campaign, the May campaign will get the credit, as it was the last touch (the open) even though they didn’t click.
Hope that makes sense and helps!
Andy