Anyway, my belief in the universality of human sin (and further still, in total depravity, but I especially highlight universality of sin because even Arminians have to reckon with this on some level) has always been a little hard to reconcile with the reality of non-Christians who do really good things. It's the neglected step-brother of the problem of evil: the problem of good.
This isn't exactly the issue that Calvin goes after here (though he certainly addresses it elsewhere), but I found this quote to be helpful regarding the sin that does and does not surface in each human:
If these [the sins described in Rom. 3] are the hereditary endowments of the human race, it is futile to seek anything good in our nature. Indeed, I grant that not all these wicked traits appear in every man; yet one cannot deny that this hydra lurks in the breast of each. For as the body, so long as it nourishes in itself the cause and matter of disease (even though pain does not yet rage), will not be called healthy, so also will the soul not be considered healthy while it abounds with so many fevers of vice. This comparison, however, does not fit in every detail. For in the diseased body some vigor of life yet remains; although the soul, plunged into this deadly abyss, is not only burdened with vices, but is utterly devoid of all good. (Calvin, Institutes, 2.3.2)