Barr signifies Barr Laboratories, while myl signifies Mylan Laboratories.  They are just two different manufactures of the same generic drug.  While each manufacturer is responsible for making sure the active ingredients of their drug meet legal requirements, bio-availability and other unique factors specific to the individual can make one manufacturer's version of the drug more or less potent.  If you had a good result with myl, and a lesser result with barr, then stick with what you know works.  For me, Barr was by far superior to Mylan.  But each person is an individual and should find what works best for them.  You can ask your pharmacy to stick to one brand or another if one or the other gives you a better result.