From the FAQ post "[How does editing work?](https://meta.stackexchange.com/a/21789)" ([current revision](https://meta.stackexchange.com/revisions/21789/28))

> During this period, any additional edits they make are collapsed into
> the same edit in the revision history, displaying only the final
> outcome of all their edits within that 5-minute period. The grace
> period ends after exactly 5 minutes, or if:
> 
>   - Another user edits the post, at which time *their* grace period begins.
>   - A comment has been added to the post since the previous revision by anyone other than the editor (ignores deleted comments).
>   - The post being edited is a question and an answer has been added since the previous revision (ignores deleted answers but *does*
> include answers by the editor).
>   - The post is a question, and it has been closed since the previous revision.

  
I think that the second bullet point applies here. There was a comment between the two edits - as you can see [on the timeline](https://mathoverflow.net/posts/481880/timeline).  

 * [Revision 9](https://mathoverflow.net/revisions/481880/9) has the timestamp 14:34:59Z.
 * There is [a comment](https://mathoverflow.net/posts/comments/1254879) with the timestamp 14:35:01Z.
 * [Revision 10](https://mathoverflow.net/revisions/481880/10) has the timestamp 14:35:26Z.

The comment might be deleted at some point - for posterity, I have posted the comment [into MO editors' lounge](https://chat.stackexchange.com/transcript/10243/2024/11/6); the timestamp should be visible there even if the comment is deleted.

Here is a related question on Mathematics Meta: [Is the 5-minute grace period suddenly gone, or is it just me?](https://math.meta.stackexchange.com/q/20045)