The cath is considered more accurate by interventional cardiologists. Many echo techs feel echoes done properly can be more accurate. In both cases, it's an estimate, not a direct measurement.
There is also the factor or regurgitation. Most of the time, when you have stenosis from calcific deposits bad enough to cause symptoms, you're also experiencing some regurgitation (insufficiency). So your heart is having difficulty pumping blood through a very small orifice, and of what is getting through, some is leaking back. A lot of heart's labor lost. Kind of discouraging for a hard-working organ.
Plus, there's the matter of your size. Are you a big guy? It makes a difference in how much a 1.3mm or 1.0mm valve aperture will affect you.
And yout heart's size is important. The more enlarged your heart becomes, the less efficiently it pumps. As your slide into stenosis begins, your heart expands liek a good muscle from the added labor. Your Ejection Fraction (percent of blood pumped out of the heart in one beat) goes up. It pushes blood through the chambers like a strong hand crushing a grape. Now it enlarges, becoming a huge Mickey Mouse hand. Over time, it has become so large that it no longer has any leverage over the tiny chamber. The chamber is like a grape getting lost in a hand so big, it can't close efficiently enough to squeeze it. The EF goes gradually down.
So there are a number of major components feeding into your symptoms. I would go after the original echo heart chamber size records and your newest echo records and see how much your LV (Left Ventricle) has expanded. This is another one of the criteria for surgery. Your symptoms are also criteria. It's not all valve size.
Best wishes,