Professional :: Motion Tweening Magnitude Of Change?

I created an animation of a wildfire progression which includes 31 hourly perimeters showing the fire spreading. I created motion tweens 24 frames long for each perimeter and tweened the alpha from 0% at the beginning of the tween to 100 % at the end of the tween.  Each tween is the same number of frames, and same frame rate, and all in the same scene. when the animation starts each perimeter is displayed for about 1 second, by the time the animation reaches near the end, the perimeters and durations are almost 6 seconds long.  I want each to display for the same amount of time.  It looks like the tween is adjusting its length to accommodate for the magnitude of change occurring in the scene.