when is 100 yards not really 100 yards?

First of all, thanks to everyone who commented both through here and ravelry yesterday. In some ways it is reassuring to know that I'm not the only one who runs into these kinds of difficulties, but on the other hand, shouldn't we be concerned that the post (and not just in the US, but in Canada and various European countries too) is poorly run that these things can happen on a regular basis?

And now back to our regularly scheduled knitting:

Of all the WIPs I showed off the other day, the Follower Scarf (I call it this because it seems that everyone is doing it right now) is growing the fastest. Normally a 1x1 ribbed scarf would seem deathly to me, but watching how the colours change and sometimes merge, sometimes contrast, is absolutely fascinating! I took some pictures this morning, out on our back stairs (which tend to be featured as the background for most of my knitting pictures - there's so much natural light and they provide a pretty neutral backdrop):



I took more pictures, click on the picture above to go to my flickr where you can find more angles, if you are so inclined.

So, now back to my question (in the title, that is). When is 100 yards not really 100 yards? I'm using two different yarns for this scarf; Knit 1 Crochet too Paintbox and Trendsetters Tonalita. Both are Aran/Worsted Weight, both come in 50 gram balls and both claim to have 100 yards. However, I used up the first ball of Tonalita long before the Paintbox ran out. In fact, as I was adding on the second ball of Paintbox I measured how much I had knit since adding the second ball of Tonalita. 16 inches. Not 16 inches of yarn left, not 16 inches to the next edge, I mean there are 16 more inches of scarf in LENGTH from the time I added the 2nd Tonalita until the time the 1st Paintbox ran out. That's a LOT of yardage, people. So, either Tonalita is far short of the 100 yards, or Paintbox is seriously generous.

How can this be? Same "weight" yarn, same weight (in grams) balls, same claims on the label... My only guess is this: the Paintbox is 100% wool, whereas the Tonalita is 52% wool, 48% acrylic. Could it be that the acrylic makes it heavier and therefore the materials used change the weight to yardage ratio? I know that another 100% wool (Cascade 220) of an Aran / Worsted weight comes in 100gm skeins and claims to come with 220 yards (hence the name). Thus, is the Paintbox, actually closer to 110yds/50gms? Unfortunately I don't have one of those cool measuring-thingys that you can strand the yarn through to count the yardage...