Here's the basic sequence of Canon's 18-55mm kit lenses from the list at the Canon Camera Museum.
2004/09 EF-S 18-55mm f/3.5-5.6
2004/09 EF-S 18-55mm f/3.5-5.6 USM Optics the same, but with micro-USM focus motor (Sold only in a few initial kits with the 300D).
2005/03 EF-S 18-55mm f/3.5-5.6 II Optically identical to the original. Cosmetic differences only.
2005/03 EF-S 18-55mm f/3.5-5.6 II USM. Optically identical to the original, but with II cosmetic changes and micro-USM focus motor.
2007/09 EF-S 18-55mm f/3.5-5.6 IS New optical design, addition of IS as well as cosmetic changes.
2011/03 EF-S 18-55mm f/3.5-5.6 III Same optics as 2007 IS, but without IS. Cosmetic changes. (not many were produced)
2011/03 EF-S 18-55mm f/3.5-5.6 IS II Same optics as 2007 IS, new IS control algorithm, cosmetic changes.
2013/04 EF-S 18-55mm f/3.5-5.6 IS STM New optical design and Stepping Focus Motor.
Following the initial introduction of both the III and IS II with the 1000D and 600D in March of 2011, the 1000D kits with the III were priced less than those with the IS II. Canon quickly dropped the III from the lineup, at least in the U.S., as consumers overwhelmingly preferred the IS version for a slightly higher cost. Any differences in tested optical performance should be attributable to copy-to-copy variation, as both held the same optical elements
as the 2007 EF-S 18-55mm f/3.5-5.6 IS. The 'III' had the element used in the IS unit statically mounted in the optical path, the 'IS II' had an improved IS algorithm in the lens' firmware. Both also had some cosmetic changes that made the rubber zoom rings more closely resemble some of Canon's higher quality offerings.