I think that is something you will find more prevalent as time goes on with manufacturers dropping support for the older bikes, whether it be oversight or intention.
Based on the chart you referenced, Avon no longer supports any tire for fitment onto a 1.85 rim in the rear, regardless of what size you select from their line. If you have problems, you are on your own. Avon can simply say you have a mis-applied their tire.
The problem is that the consumer has no way of knowing if the tire design will still function correctly with the narrower rim. We don't know if the designer/engineer made the chart, or a front office type. Either way, the company has narrowed their exposure to customer claims.
I have a PDF from Metzeler stating that the 3.25x19 and 4.00x18 Lasertec and the ME77 4.00x18 can be applied to a 1.85 rim width.
I'm have problems with their website that won't let me accept their agreement on line right now. But, I got the PDF chart from their "technical"
section previously.
There isn't much you can do when a manufacturer decides not to sell tires for your bike, except switch to another manufacturer that does.