Because it's the SAME memory:

The New Intel Core Duo and Core Duo 2 Powered iMac models have two memory expansion slots that use special 200 pin Mac spec DIMM DDR2 type modules. The 1.83GHz models come from the factory with 512MB installed, the 2.0GHz and faster models have 1GB. There are two memory expansion slots that can address up to 1GB each in the 1.83GHZ machines with a maximum total potential being 2GB. The 2.0Ghz and faster Core Duo 2 models can expand up to 3GB maximum.

www.macramdirect.com
or
www.datamem.com

both of those sites, plus crucial will stock the memory you need.

___________________
-Chris
"the night moves me" NIGHTMOVES.ME nightlife+lifestyle photography