No Senator or Representative shall, during the Time for which he was elected, be appointed to any civil Office under the Authority of the United States, which shall have been created, or the Emoluments whereof shall have been encreased during such time; and no Person holding any Office under the United States, shall be a Member of either House during his Continuance in Office. (Constitution Article I, §6, para 2)
My interpretation of this is that a senator, once elected for a six-year term, cannot be appointed to an Executive Branch office during that period, if the office was created or its salary increased since the Senator took office. The obvious rationale for this would be to prevent congressmen creating lavish appointments for themselves and then transferring into them.
The Attorney General is a Level I position; the salary for which (along with those of what looks like most/all of the Executive Branch) was increased to $205,000 in January 2016. A similar increase also took place in January 2015; I presume it's an annual occurrence.
Sessions began his Senate term in January 2014, and it extends until 2020. Why is he, as a sitting senator, allowed to take up the AG office prior to 2020?