If you're are talking about the range of ISO sensitivities available then it's simply a choice by camera manufacturers about how much gain (amplification) is applied to the sensor data. The highest settings are usually implemented in software by just doubling all the numbers after analogue to digital conversion, so it's entirely arbitrary how high they can be made to go.

The Canon 600D goes from ISO 100 to ISO 6400, whereas the newer 100D goes from ISO 100 to ISO 12800.

Lots of people claim that Canon have been using the "same 18MP sensor" in all APS-C models since the 7D. This conjures up images of a warehouse full of 18MP chips manufactured in 2007 that Canon keeps dipping into with each new release. In fact each sensor is different, but the differences are incremental based on the same basic design, hence the difference in ISO range (and other factors) between the 100D and 600D.

It's tempting to say that as sensors get better at handing noise, manufactures have increased the allowable range accordingly. But ISO ranges have expanded far faster than camera performance. The 20D only went up to ISO 1600, it's 2013 descendant the 70D goes a whopping three stops (x8) higher.

In essence it's become an arms race between manufacturers as the higher setting tend to suggest a camera is better in low light. Both the 100D and 600D offer an "expanded" ISO setting of 12800 for the 600D and 25600 for the 100D. It's hidden behind a setting so manufactures can claim a wide range, whilst at the same time preventing anyone from using it due to the very poor quality of detail captured with this setting.