Why would I lose the ratio of events over sessions?

Hi there

I built up a series of dashboards to plot engagement with the platform I look after and used ratio (what % of user perform an event?) style queries to offer a % view of engagement vs sessions. They were all saved to a dashboard yesterday but overnight the charts appear to have been updated to be a count of unique users? 

In doing so, because the global session count is so much higher than some actions on the site I have lost the sense and seasonality in the more granular parts of the platform? 

Is this a bug? Why would the charts update themselves? All colleagues have denied updating the charts too ☹️

Thanks, Ade