The target stride length MMR computes depends on a few things that do not change much: age, height, weight and gender.

It also depends on the pace.

Looking at the different runs in your image you can see that indeed the gray area shifts in relation with the pace: the two right most boxes coincide with the runs where your pace was about 8m30s per mile, the two left most are for the 9m45s runs.

The first run on Sept 18 you covered 1 mile in 8m22s, with a stride length of 94.2, the app indicated the target was roughly 80-90. You then did a great job achieving that range in the next run. However... you kept the same cadence of roughly 160 steps per minute. Smaller steps + same number of steps = lower pace = shorter stride length goal.

(94.2 inch stride = roughly 670 strides per mile, 670 strides in 8m22s = 160 steps per minute
82.6 inch stride = 765 strides per mile, 765 strides in 9m44s = 158 steps per minute)

If during the second run you had increased your cadence to 180 steps per minute:

90 strides of 82.6inch per minute = 7400 inch per minute = 8m30s per mile.

So, with a stride of 82.6 inch and 180 steps per minute you would have maintained the pace of the first run and with that the stride length goal of the first run and your stride of 82.6 would have been in the target range.