The first releases of all cores tend to be less 'refined', hence run a bit hotter. The original Palomino XPs ran a tad warmer than equally clocked default tbred cores (your 1800+ came in the palomino and tbred A flavors).
Although the tbred A cores were made with a newer process and ran more efficiently, the actual size of the core was smaller, so there was less surface area to transfer heat. As a result, any advantages gained from the new process were lost in the shrinking of the core's size. Some earlier tbreds (all the tbred A cores were 'early'/first tbred cores made/released) actually ran hotter than the palominos at the same clock speed and cooling.