The thing to remember about focal length is that it is only a measurement of one aspect of the lens, the effect it has on the image you record is down to the sensor or film size, not the focal length. A 10-20mm lens is always going to be a 10-20mm lens whatever sized sensor or film is behind it, and whether the image it casts is larger or smaller than it can record.
On medium and large format systems 50mm is considered a wide, on a 35mm camera it is "normal", for a DSLR with an APS-C sensor it is a short telephoto, while on a digital compact camera it was would be a very long telephoto. The one thing they all have in common is that they the focal length is 50mm regardless.
I think the biggest sources of confusion about focal length comes from DSLR cameras being a successor to 35mm film, and in most cases retaining the same lens mounts and so lens compatibility, that people try to relate the focal length to a 35mm system giving it two meanings. This is why there was never confusion about the different effects of a focal length between other systems, because they were treated as separate and not compared in the same way.
It also does not help that even amongst DSLR cameras there is not one standard sensor size: Nikon ones are 2/3rds the size of a 35mm frame, Canon uses 5/8ths the size, while Four-Thirds system sensors are half the size. And that is just a rounded average. The 35mm equivalent field of view for Sigma's 10-20mm lens is 15-30mm on a Nikon, 16-32mm on a Canon, and 20-40mm on a Four-Thirds camera.
It would be confusing enough to consumers to have to understand two different systems of measuring lenses if Nikon and Canon used the 35mm equivalent figures for their DX and EF-S lens ranges. For companies like Sigma they would need to use different names for each mount they support.
Thankfully that manufacturers always giving the correct focal length is something that actually helps us because we do not have to worry about such things. A 50mm lens on a D80, for example, will give the same view whether it is a DX or DC lens or not. It would be a lot more confusing if you had two 50mm lensed and had to start wondering whether one is normal and the other a telephoto.
As far as I am aware the Sigma 12-24mm lens is the largest rectilinear (non-fisheye) lens you can get for a 35mm frame camera. A 10mm lens on this format would, as Joe said, be extremely expensive and suffer distortion problems.
Unless you need to work in both APS-C and 35mm systems the best advice is to ignore the 35mm equivalent numbers and just learn what the difference is between different focal lengths on your camera. Then if you do need to relate to a 35mm size, say while reading an article talking about film camera, you can convert in the other direction from that into the system you know and use by dividing the focal length by 2/3rds (for Nikon).
35mm is not in anyway "normal" or "right" that everything else should be considered a variant of, it is just one of many different sized systems used in photography, and there is no need to always think and relate in 35mm terms.
Michael.