When you use sprintf() with a %d format, the number is converted into the minimum number of characters needed for it. For 1, 1, and 7, that is one character, so for those three the resulting strings are all the same size. But then you reach 13 and that takes two characters to output, so the resulting string is longer than the ones before. You are using TestLbl(k1,:) as the destination so you are trying to write a row which is longer than the existing rows. You cannot have rows of different lengths in a character array.
You need to assign to TestLbl{k} (a cell array entry) or else you need to ensure that the strings are all the same size such as using %3d instead of %d. %3d means to use at least 3 characters, so for example space-space-7 for 7.
