Seven-segment display seems to draw little current

First of all +1 for measuring your display. Too often we get questions here about "I saved money by buying cheap from Alibaba but it don't work. Now please spend you valuable time helping me out".

These days LED's are very efficient. They no longer need the 20mA the first generation required. 1mA or less is not uncommon.

So, yes, you drive these straight from an atmega328.