It depends what the "tiers" mean. If someone is tier 1, then that means that they have god-like mastery over the force, so Yoda and Sidious would both fit in, though Sidious has more knowledge of the force, and is just straight-up more powerful. I think this is specifically refering to their effectiveness in combat through the force and lightsabers skills/physical powers. So people of varying power, based on their application, can be in the same tier.
tier 2 is said to be the best that can be achieved by dedication and practice (I disagree, since Sidious' midi-chlorians count was much lower than those far inferior to him, but I digress), so that means that kenobi's skills in combat would be more refined that Anakin's, though Anakin is still more powerful, skilled, a better tactician, and so on.
Jen has admitted before that Anakin is the most powerful, but his idiocy costs him tier 2 level mastery. We know this to be true: Anakin found it difficult to meditate, couldn't let go of his connection, and often became passionate when jedi (or even normal people) would stay more or less calm. So him being incapable of utilizing the force with the same level of effectiveness as Kenobi, or Marek or other weaker force sensitives, makes perfect sense.