I purchased the 21" rolling carry-on and found it to be much smaller than advertised. I bought this to replace my 21" Roll Aboard bag which actually measures 21". I can easily fit the Rolling Carry-On bag right inside my old bag. Side-by-side, the size difference is really stunning.
Here are photos comparing the two bags: https://www.dropbox.com/sh/gfi5waqw8ko8ozm/AAAriFwTdshnDM_TjGqoz5Vea?dl=0
When I contacted customer service, they sent a photo of how you can get a 21" measurement by really stretching the fabric of the bag just right and they won't cover returns. Return shipping was nearly $50 so I'm stuck with the bag for now. I just really don't want other people to end up in this same situation.
If you're happy with an 18" bag, this bag should be great. If you're hoping to replace you're old 21" bag, look elsewhere.