Six Does Not Equal Six

Huh?
Let me explain.
I was diligently working on my Marl Yoke Raglan last week at Knit Nite. I was SOOOOO close to the end of the body and place to join the sleeves that I kept knitting so I could say I had the sweater that far.
Since the shawl I am making for Evelyn is a bit too complicated to take to Knit Nite (I tend to talk a lot there.), I wanted to have the sleeves ready to work on. I dug out a pencil and my copies of EZ's Opinionated Knitter, and Knitting in the Old Way and started calculating. I figured out how many stitches I needed to cast on, how many rows it would take, how often I would have to increase if I did it this way or that way. And so on.
I was so thorough.
Finally I cast on with a set of Plymouth bamboo double point needles and knit about 6 rounds or so.
Then something made me pause. The ghost of Elizabeth Zimmermann perhaps?
Anyway, something made me stop and put away the size 7 needles tips that I had out. And while I was at it, I checked the tips I had used for the sweater body. The needle gauge I was using showed the size 7 slot loose and sloppy.
But wait! So was the size 6. I had supposedly been using Size 6 in my Harmony Options. I dug out another needle gauge and the size 6 fit perfectly.
Hmmmmm.
Then I looked at both gauges. One size 6 was a 4.25mm. the other was 4.00mm. I checked the double points I was using for the sleeve. Size 6 all right. but 4.25 not 4.00
Sigh.
I just knew if I continued with the the sleeves with size 4.25, the gauge wouldn't match when I joined the sleeves to the body. So I ended up logging on to KnitPicks and placing an order for size 6 4.00 mm double points and also a 16 inch circ (and some lace weight yarn, and books. Oops, the mouse must have slipped.) I frogged the bit of sleeve I had started and waited. Yesterday my package came--in just 5 days!
Maybe now I can knit with peace of mind.
Or not.