This is a bit of the pen testing- you can see none of these made it to Z without skipping, if not stopping completely. You can't see it super well here, but you might notice the last line is darker and thicker. This variation will often happen within the same pen between words or sentences.. going darker or lighter, thicker or thinner. I can't trust them to write consistently. So I have about 4 semi reliable pens out of 20.
ElliottI wondered about that too, but I've tried them on everything from cheap printer paper to clairefontaine, rhodia and Tomoe River. I'm at a loss. A couple of pens write really well 90% of the time but start skipping and writing lighter eventually and the rest of the pens do it all the time!