But you see its not necessarily in entry order.
What you are getting is the lowest ID for EACH group of categories, which in this case may be the earliest entry for each category.
You have to remember that once you GROUP by a value then you are summarising all records for that group into a SINGLE record. So by grouping by Category, you need to work out what you want to do with any other fields you wish to summarise.
In this case weve taken a MINIMUM of all the ID's for each group, and sorted by this.
Personally, I would be adding a 'Date Created' field and using this as it's more meaningful that using the ID.
Hope this makes it a little clearer.
Cheers,
Andrew