Site Name/Slogan/Primary Links Layout Breaks for Some Combos of Character Lengths

When displaying primary links, I've noticed some weirdness in how the header is rendered depending on the character lengths of the site name and slogan.

It just so happens that my site name is 16 characters (one 7 character word and one 9 character word) plus a space. In the images I've attached I've used "Abcdefg Hijklmnop" with "Short Tag Line" as a slogan just to demonstrate what's going on.

With 7+9 characters in the site name, the header seems to be slightly offset from the rest of the page (see image).

If I add a character (7+10), everything displays just fine (see image). (Where the character is added doesn't matter. 8+9 works just as well.)

If I subtract a character (7+8), I get an embedded scroll bar in the header (see image).

The length of the slogan seems to have minimal effect on this. With a longer slogan, all cases exhibit the same behavior. With a shorter slogan, all cases except 7+8 exhibit the same behavior. 7+8 with a shorter slogan goes back to displaying correctly again (see image).

All of these problems only appear if displaying primary links. Without primary links, all cases display fine.