How Did That Happen?

I swatched on 3.25mm needles (KnitPro Harmony tips) and got 6 sts/inch
I swatched on 3.00mm needles (KnitPro Harmony tips) and got 6.5 sts/inch

So, I cast on for a cardigan using some heavy-ish (more sport-weight than 4-ply) yarn from Fyberspates, using 3mm needles for a moss stitch bottom band.  From my 3.25mm swatch (the needles I was planning on using for the body), 253 stitches, take away 13 stitches for the overlap on a wide button band = 240 stitches, which equals a width around of 40", which gives me 2-3" ease around the bust (depending on hormone levels and what I've been eating/drinking).

So, how come, when I measured the bottom band of which I'd knitted 16 rows on 3.00mm needles (because, with my years of knitting experience, I expected the moss stitch to be more or less the same tension as stocking stitch on 3.25mm needles), it came out at 45 inches?  I know that moss stitch (or seed stitch if you're in the US) can be a strange beast, but 5 inches bigger? 

The yarn's now back in balls (and I've glared at it and had words with it, so it knows I'm displeased) and tomorrow I shall swatch it on 3.5mm and 3.75mm tips and see how it looks (I think 4mm will be a bit too floppy), before doing some maths to work out which size from my basic "school cardigan" Sirdar pattern I should use (it really is a school cardigan pattern - I've used it several times for DD1).

I've also got to decide if I'm going to live with button and buttonhole bands striping obviously (the yarn's variegated and I'm going to alternate balls every couple of rows) or use dpns in a smaller size and knit them as I go.

Did someone say that knitting's a relaxing hobby?

I'm now going to do a bit more of my Granny Stripe crochet blanket.  At least I know where I am with that.