In recent years, some tzitzit string manufacturers have begun making different versions for different types of tying. The reasoning behind it is a bit complex, but I'll share it because some readers might find it of interest: There's stiff competition among tzitzit manufacturers, perhaps what you could call a wholesale price war, since tallit and tzitzit sellers like us use at least a few hundred sets per month, so a shekel here and a shekel there makes a difference, and we try to pass those savings on to our customers.
To make their prices competitive, some of them have resorted to tricks to save on costs. Since the price of wool is significant, some make their shamash strings a bit shorter, while others make all of the strings a bit thinner (i.e. even though it says "thick" on the package, one company's "thick" might be slightly thinner than another company's "thick").
And then there are two or three companies that came up with a fair, legitimate, transparent way to save on costs in a win-win fashion. All tying customs require that one string, known as the shamash string, be longer than the other three; however, for some tzitzit tying customs (namely Ashkenazi, Yemenite and Sephardic 10-5-6-5) you don't need as much shamash length as others (Chabad and Sephardic 7-8-11-13).
Among retail customers in Israel who do their own tying, the vast majority are going to tie either Ashkenazi or Sephardic 7-8-11-13, so to make things simple for consumers, the tzitzit manufacturer we currently prefer labels the tzitzit strings "Ashkenazi" or "Sephardic," but the only difference is the length of the shamash string. When we finish tying, we trim the tips with ceramic scissors, so essentially if we were to use so-called Sephardic strings for Ashkenazi tying, the only difference would be that we have an extra inch or two to trim off of one of the strings.