First, let me preface this by saying I have never done a sleeve but I do read a lot! My thought is that you are to increase at the beg/ end, creating a "seam line". I just glanced at EZ's Knitting Without Tears, and her sleeve instructions have you increasing on either side of a 3 stitch underarm line, the only difference would be that your increases would be next to each other instead of separated by a few stitches.
The wording in your pattern "when available" would certainly would have me muttering some not-very-ladylike grumbles for sure!
I'm not obsessed, I'm focused.