It does not Trim by string, instead it trims by character, so all C,o,u,n,t,r,y CHARs are trimmed from the beginning, until it hits first non-match.
In your first case, it hits 'a' and stopped there, while in the second case it stopped at L.
string.TrimStart Method:
http://msdn.microsoft.com/en-us/library/system.string.trimstart.aspx