You saw my comment for that run last week, - I loved the carb total you got, and I loved fueling opportunities!!
Just think of it as a positive. Fueling opportunities! Every low or drop is a chance to give your body what it needs. Those are great things.
A flat 90 gives you less of a chance to do that! 
We don’t want it to be a crazy spiky up and down rollercoaster, but a nice gentle slope up and slope down. Every “up” is a chance to take a small bolus, and every “down” is a chance to take carbs.
Awesome! You are totally normal!!
And I am so excited to read your comment there! You have figured it out!!!
IC’s get crazy when running!
Yesterday my IC was 164. Some days it is 200+. For very long races it goes over 300. There are days when it was infinite, I kept taking carbs with no insulin for 20 miles.
Quick tangent, very useful to read!!
See reference post! ===> contraction mediated pathway.
This 
one post is a good one to read, @LarissaW!
As a general rule, you should try to find the starting point for bolusing while running, like after 1 or 2 miles, and then just start reducing it each few subsequent miles.
It is not an exact calculation, just an estimate.
As you said, it won’t always be consistent, but just find the initial amount that works and then take less and less for subsequent carbs the longer you run.
So just as an example, if your first few miles you take 0.50 units for 20 grams, then after 3 more miles, maybe you would take 0.40 units. Then a few miles later you would take 0.30 units, and on and on like that.
If you hit it too hard after 2-3 miles, the next adjustment would reduce it more. If you hit it too soft after 2-3 miles, the next adjustment reduces it less.
Analyze and adjust for each run.
All make sense?