I was contacted by a customer who had explained that they were not seeing new Minor Versions after "X" number of versions. They told me the version was being stuck at 1.06 and that version kept being overwritten. I was a little perplexed, so I asked 3 of my co-workers (4 of us total) to join a co-authoring session and see if we could reproduce the issue. We were indeed able to reproduce this and it is by design. I will explain.
First, a new version will not be created till everyone has left the workbook (the lock on the file has been released). SharePoint can then save that version to the library. So, if you are alone in a workbook, make a change, flip to Reading View (leave Editing View), you will immediately see a new version. But, if there are multiple people in the co-authoring session, everyone needs to leave before you see the new version.
While people are in an co-authoring session, you will see the Modified column is updated about every two minutes while multiple people are working on the file. In our testing we found that the last person to enter the workbook will get credited under the Modified By column. So if Tom, Ben, and Adam join a co-authoring session at 8:00:00am, and Taylor joins a second later at 8:00:01am. He will be listed as the Modified By Author.
So, if Taylor writes “Hello” and does nothing while everyone else fervently types away till 8:01:59, Taylor will still be listed under the Modified By column when the version is updated at 8:02:00 since he was the last to join the workbook (even though he did not make the last edits (Tom, Ben, and Adam edited the cells last)). See the screenshots below to see the Modified column and Modified By column change every two minutes.
*During the testing, I had people leave the co-authoring session and re-enter, making notes of who the last person to enter was. Always making sure two people were editing at the same time (co-authoring). As you can see, the Minor Version is updated every two minutes by the last person to join the co-authoring session.
I hope this clears things up.