I'm not certain about the difference between 'opened case' and 'return request', as they both technically became a case in the resolution centre, and while I have had a couple of requests opened for not as described, they were a little while ago and I can't recall the wording (mine were also resolved without the request being escalated to eBay).
It may be just how the buyer goes about opening the request (they can use the "return this item" link in eBay, or they can go to the resolution centre and open a request from there).
Currently, a return request will result in an automated defect if the reason selected by the buyer counts as 'not as described', if they select a change of mind reason (eg doesn't fit, don't want it etc), then a defect won't be issued. The defect will only be removed if the request is escalated and closed in the seller's favour, or if the buyer closes the request and selects the option to indicate it was opened in error. (The grey dots in your screen shot are defects which were subsequently removed).
If a buyer doesn't close a request, then it's supposed to time out with no further action from eBay (but the defect isn't removed).
In saying all that, in a couple of weeks all of these defect-causing issues won't matter, as eBay is revising the entire system....
Under the new system, only two things will result in a defect - cancelling a sale due to being out of stock, and a case closed without seller resolution (where a request is opened, escalated to eBay, and found in the buyer's favour). So low DSRs, the act of opening a not as described request, or neutral / negative feedback will no longer cause a defect at all.
