Selecting odd ISO

When I set ISO to auto, camera sometimes selects numbers like 2200, 1850 etc. These are not standard ISOs like 100/200/400/800/1600/3200/6400 etc. Is it only possible in auto ISO mode? When I try to select ISO manually I see that I could only select full multiple numbers.

ISO isn't as drastic between increments as aperture or shutter speed. Not a huge difference going from 200 to 225 for example. To be honest, most people set it to the less noise possible and forget it. I believe the odd ISO numbers are only selected only in auto or "P." In most situations, it's going to take too much time figuring out the exact ISO setting to use with very little difference in results.

That frustrated me when I first got my D3100. I believe you can set parameters, or better yet, turn off auto ISO and set it. Even with flash, it will select crazy numbers. I don't think Nikon put low noise as a priority in auto mode.

It has been a mystery to me why Nikon tends to select very high ISO even in quite bright light situation! It seems totally counter intuitive. I searched on web and found that a lot of people were confused about this.

I have now started using manual ISO but still now I was using auto ISO and lot of my shots were showing ISO much higher than I would have set manually.

I found following discussion quite useful.
D3100 Auto ISO issues - Reply from Nikon: Nikon DX SLR (D40-D90, D3000-D7000) Talk Forum: Digital Photography Review

and some 2/3 star reviews at amazon.com

I always set ISO manually because I want to know exactly what it is and that makes it easier to work around apertures and shutter speeds in relationship to that. If it varies all the time then you don't really learn how to work with particular ISOs since the camera just does it for you. I almost always shoot at ISO 200, for example, unless I really have no choice indoors but to increase it but would not want to go beyond 800. I'm sure with many of the better cameras it is ok to go even to 1600 with clear results but I guess I like to make sure that any noise is minimized as much as possible. But in answer to your question, yes the camera can set nonstandard ISOs automatically but you cannot because it is based on stops (200 is one stop more than 100, etc.).

There's a menu setting that allows you to set the ISO sensitivity increment at either 1/3 or 1/2 step. The 1/3 will give you additional values, but nothing like those oddballs.

The way it's metering determines what auto ISO it chooses for the scene. Center weight is more accurate in certain situations for example. ISO adjustment is a last resort when you don't have a ton of light to work with and free handing. You should ALWAYS set it manually to the cleanest setting. The computer can't determine that. It just chooses the best exposure for the shot regardless of noise.