Garmin Forerunner 265s is consistently overcounting steps.

See support.garmin.com/.../

Done all that, checked them all and problem persists. I even performed factory reset moments ago, I went to the kitchen and back, + 100 steps (manually counted 30).

can it be defective accelerometer or something?

I even checked the wrist to be correct, weight, height, age. 

I had (and still have) the opposite problem. When you look at your watch for a long time while running, your hand kind of compensates for the impact of your legs and because of this, there are dips in the step frequency graph. Try walking to the kitchen, bending your arm at the elbow and constantly looking at your watch - maybe something will change. It seems to me that the error per day may not actually be catastrophic, it's just that under certain conditions the error is large, and with uniform monotonous walking the accuracy is normal.