Darn, that's really a mystery, sorry I don't even have a guess!
Oh, also Citabria, on those Martin M-600 soprano strings, just a word of caution-- they recently changed their packaging on these strings, so that instead of their older blue-and-white color theme, they now have a nice light green background with some pink, orange, and yellow flowers across the front. I recently got a few sets of these newly packaged strings, and I discovered (as I think others here have too) that the four separate envelopes that you find when you open up the main packaging have MIS-LABELED the strings contained therein, so that it's VERY confusing to determine which string goes where! Honestly, I'm disappointed that the folks at Martin would let this happen. The good news, however, is that the strings ARE correctly designated on the reverse side of the main package, so if you follow THAT, you should be okay! But, to clarify, as you hold the uke out in front of you with the strings facing you, the left-most string is considered the fourth, or G; the next from the left is the third, or C; the next is the second, or E; and the far-right string is the first, or A. Match the string "gauges" shown on the back of the main package with those indicated on the separate envelopes. Hoping this helps!