There is something really "whacked out" with that SC flot chart This is what I find on multiple sites IF SC chart is anywhere close to valid I'm at a loss as to why
So I bought some SGOV on 13th at 100.46 and woke up yday with a price at 100.00. What the heck, it is not the 1st when they pay interest. As you correctly said, I am sure it is a capital gains distribution for the year. So I unknowingly bought the last day before Ex-Div date.
One thing CS does not do a good job of that IB does an excellence job of is showing dividend Ex-Div date, the amount, and the pay out date. At CS I got no information, but on the 20th the dividend will show up. Years ago I had started a Div Accru column where I could log the dividend and have it show in my NAV. I then move the number to Div on the payout date. I have added "Events" to almost all of my long term daily charts, so I know when an Ex-Div date and amount has happened.