I was just analyzing my VBTLX holding at VG and noticed that my cost basis for this holding is slightly negative (small total loss), however, when examining it's rate of return over the same time period (on VGs "Balances over time" screen), the ROR shows a small positive value. I don't necessarily understand this apparent discrepancy. Could it be due to monthly interest payments being reinvested? And such reinvestments are accounted for in the ROR calculation, but not in the cost basis computation, where the latteronly factors in the price per share, but not the fact that they were "free" (monthly interest payment) shares?

Or is there another explanation?

Thanks in advance.