Are they followed by other entries???
Cmd-z will only undo the latest actions taken anywhere in Lightroom.... not just Develop settings.
Ctrl/CMD Z wont work on the image history panel, simply go to the last entry you wish to be applied and click on that. The rest of the entries will be undone.
Have a look at the "Track image Adjustments" on this page: Adobe Photoshop Lightroom 4 * Manage image history and snapshots
yes they are followed by more edit adjustments. That concerns me now that I have been doing cmd z, I wonder what I have been undoing.
I suggest you may have done something to cause these entries but really they are of no importance if everything else is ok.