Discrepancies between Meta Events and Custom Conversions

We've got a couple Meta accounts where I've setup custom events through GTM for them - which appear to be all working correctly.

To "expose" the fields in reporting columns and Looker Studio, I've setup custom conversions for each event. Each conversion is set to use that event, with the url "/" to capture all traffic for the event, which to me should make the conversion and event identical in number.

Within the Events Manager view, through the Data sources and Custom conversion pages the numbers aren't the same. Same issue when viewing from the Ads Manager, and looking at the event vs conversion columns values. I'm only measuring from the date the custom conversion were made, so they should be identical.

Is there something I'm missing with the events and conversions not being the same?