While looking at the code for the calculations of extraterrestrial radiation get_extra_radiation() and crosschecking it with literature, I noticed that for both methods 'asce' and 'spencer' the solarposition._calculate_simple_day_angle(doy) function is used. This function basically just calculates (2. * np.pi / 365.) * (doy- 1), which is correct for method='spencer'.
But I think for method='asce' it should just be (2. * np.pi * doy / 365.) - without "- 1" - as described on page 9 in "J. A. Duffie and W. A. Beckman,  "Solar Engineering of Thermal Processes, 3rd Edition" J. Wiley and Sons, New York (2006)"
I just happen to have this book in front of me, so I checked on page 9, and there is indeed a difference as noted w.r.t. the pvlib-python code.  One formula uses n, the other uses n-1.  This difference is probably not significant, but perhaps it should be fixed just so it is correct.  An issue on github would be the best way to raise this.
Minor mistake in calculation of extraterrestrial radiation (method 'asce')?