> In the first code, 10 spaces were added before 'G1'
That's incorrect. 8 spaces are added prior to 'G1'. From the documentation, the asterisk and number pair defines the minimum number of characters to print including the number of characters in the string. In this case, G1 contains 2 strings so there are 8 spaces added.
This solution uses an anonymous function addSpace(n,str) where n is a positive integer and str is a character row vector. Call the function within fprintf inputs to add n spaces prior to the string.
addSpace = @(n,str)[char(32*ones(1,n)),str];
fprintf('%s %.1f%s %.1f\n', ...
addSpace(10,'Average_1:'), rand, ...