Its possible, but I don't think thats the only thing, the reason being that they are listed via Realtors (usually) and I can sometimes find them on other websites (e.g. Redfin) or in the local real estate flyer booklet, but it simply isn't on Zillow for some reason.
I wish I had a recent example, but I tested this theory multiple times when I was house hunting, and I don't think I'm hallucinating the issue.
The following video explains some of the possible discrepancy due to pocket listings not appearing in Zillow. It also suggested that realtor.com had more listings in its database in some neighborhoods.
There are multiple MLSes and Zillow only has deals with most of them. It's possible you're in a region with prominent "local MLSes" that decided Zillow wasn't paying them enough for access.
I wish I had a recent example, but I tested this theory multiple times when I was house hunting, and I don't think I'm hallucinating the issue.