Not sure where you get Lannan is a 5 inning pitcher. I'm not a huge fan of the definition of quality start because of the ER aspect, but Lannan has a career 54% QS (48% in 2010 and 2011). That's at least 6 full innings. So, more often than not, he's not turning the ball over to the bullpen before the 7th.
edit - for his career, he's 5.85 innings per game started. MLB average is 5.9. Lannan's not quite 6, but very close - 1/2 an out from 6. By contrast, Jackson is 6 on the button for his career, and 6.1 (one tenth) last year. To be frank, last year was an off year for jackson.