When you are close to level 65, 75 or 85 you get sometimes better missions, so you should always take a look at the basic exp they grant. When you reach 67, 77 or 87 you cease to get the lower exp missions. For example, as praiseluka said, when you are level 85 the largest part of your missions grant 20k and some of them grant 25k, when you hit 86 the number of 20k missions become lower and at 87 you get only 25k. The same happens at lower levels.