The zip files counted there are part of how OmniFocus stores its data internally. We show you this count so that if you run into any trouble, our support team can use this information to help out. Usually, you can safely ignore this number unless it grows exceptionally large (many thousands of zip files), or you start seeing other problems (slower sync, for example).
If you have any more questions, feel free to follow up here or shoot us an email.
To expand a little on @tekl’s comment: the lesser this count, the faster sync runs.
The reason for fragmentation is either
not keeping all Your devices regularly synced, or,
if You switch devices but have the lost / gotten rid of device still in your sync list.
If you go to Prefs/sync and click on the little gear icon next to the “sync now” button, you will find a list of devices with their last sync date and the option to unregister the missing sync.
Thanks for the extra information. I checked out the list of devices and for some reason there were 2 entries for my Macbook Pro. One was bold which I assume indicates the device I am using now, and it last synced last night. The other entry was last synced 3 days ago.
So I deleted the older one and synced the Mac, iPad and iPhone. My list of zip files has now dropped to 23.