Oh ok, that could have explained the “shift”, but that's not the cause then…
Yet another possible reason for the activities not counted: do those that aren't counted have HR data recorded? Because Strava uses HR as one of the effort score factors… but just guessing, as you’ve mentioned recording with a phone, and not all phone Strava users have a HR sensor paired with it. Try to toggle on Perceived Exertion in the Edit Activity screen to use it if your heart rate recording was missing.
Besides of that - is there any difference between activities that do and don't show in the graph? Or the other way around, what do the omitted activities have in common?