On gauge and needle material

I have been involved with knitting some item for this event where I will be teaching knitting basics. I decided on a cowl and started working at it with sime unknown very gray yarn I had in my stash (looks like an undyed, so it's probabily an old ball of organic yarn from Silke by Arvier) and 5 mm needles. I started out with an Addi Turbo short circ, but I quickly grew bored by the short tips and this morning I switched to a 5 mm Denise interchangeable.
WOW! The gauge immediately changed. I am not sure if you can see it from the crappy pic (click to espand it), but after thechange, despite the fact that I knit much faster and more comfortably, the gauge must have dropped one or two sts lower (on 10 cm/4"). Not a bad thing: the cowl is now distinctly softer and the wafer texture much more pronounced, but I did not expect such important difference knitting in the same stitch pattern and in the same yarn with the same needle size, just in a diferent material.
(To learn more on the October 15 even for the homeless, check out this post.)