It doesn't have to be "gusty". Most shooters don't understand winds effects on a bullet path or realize it doesn't have to be hitting you in the face to effect the bullet. You can have wind between you and the target that you can't feel. That's why the top shooters use wind flags and have learned about it. This small amount of drift shown by the OP can be just wind.....even at a 100 yards without being "gusty".
I did not say it did. It has to be consistently enough from one direction (or add up to that) , it has to be a left to right at 90 degrees for the charts (or a breakdown of what the drift component is at other angle)
What I am saying is you have to walk you7 target out there and you have a pretty good idea doing so how much wind, what direction, how many yards of it total and if there is a variation.
Ergo, you don't need flags at 100 yards. Most times it zeroes out or is not enough or its not from enough of an angle to matter.
300 yards and more, yes we are talking a nice steady drift fro9m left to right will begin to play in and have an affect. Not 100 let alone someone who has shooting experience as does the OP.