Official vs actual carry-on luggage dimensions (USA)

On Jan 3, 7:40*am, Craig Welch wrote:
Tchiowa said:

On Jan 2, 5:02*pm, "William Black"
Name the airports where the regulations are not enforced.
Changi. I fly SQ regularly and usually first class. I have carried
multiple, large, heavy bags on occasion.

Ditto, amongst others.

Cathay at Hong Kong is the same.

A couple of times at Brisbane, my carry-on was weighed after x-ray,
and the security goon told me that I couldn't take it on board. I
said to him "that's a matter between me and the airline", and
proceeded. He did nothing.

A few years ago I was passing through Frankfurt. When I checked in on
as SQ flight, my carry-on wouldn't fit through their "gate" on the x-
ray machine. They told me I had to check it. I went back the SQ
counter and an agent went with me and told them the bag was OK. I
carried it on.

My original question was (paraphrased):

I'm shopping for a carry-on bag... most of the manufacturers' sites
give the dimensions as 22x14x10". ...the official maximum size for a
carry-on bag is 22x14x9". Can anyone explain the discrepancy?

I didn't get a chance to ask about this at the airport, but I think I
found an explanation. Most of these bags are described something
like, "Expandable for extra packing space." I believe larger
dimensions reflect the expansion, and the bag will still fit in a
carry-on space unless it's packed so tightly that it can't be
compressed.