Does the traffic always move faster in the “fast” (left) lane than in the slower

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I live in northwest Tucson and work about 20 miles away in southwest Tucson. Each morning I get on Interstate 10 by my home and travel south to Interstate 19 which takes me to an exit near where I work. The total trip is about 20 miles. From where I get on I-10 to I-19 there are three southbound lanes and, on the other side of the divider, 3 northbound lanes. Lately I have found that I can travel a little faster, with fewer stops, by staying in the right lane and dealing with the traffic merging on from the on ramp and the slowing due to people exiting, than driving in the middle or left lane. For some reason, those two lanes are always stop and go. Coming north in the evening the middle and left lane move considerably faster than the right lane. Is this just true of this one section of I-10 in the morning or have any of you experienced the same thing? Any ideas as to why this occurs?