Hi Djobydjoba,
I had to think about this for a minute... I put together an example to make better sense of it. The result is that the reason for the difference is the market timing of when you make contributions into the portfolio. The price version of the overlay graph isn't affected by market timing as far as comparing portfolio to investment. However, the value version is affected by market timing of external contributions.
If contributions are timed well in the portfolio, compared to putting the same money in the overlaid investment, the value version of the portfolio will look better than the price version of the portfolio. The opposite is also true. If there are no contributions, the price/value versions will look the same. Here are some examples to illustrate. Assume you look at a portfolio and index when there are no external contributions: the price/value look the same (call this case 0). However, if you add external money when the portfolio is relatively lower than the index investment (call this case 1), the result is the portfolio value moves up compared to case 0. If you add external money when the portfolio is relatively higher than the index (call this case 2), the portfolio value will be lower than case 0.
- Case 1
- 1fund.png (9.61 KiB) Viewed 178 times
- Case 1 - Price version
- 1fund_price.png (8.3 KiB) Viewed 178 times
- Case 1 - Value version
- 1fund_value.png (8.67 KiB) Viewed 178 times
You can see in Case 1 the value version of the portfolio has performed better than its price version. If we reverse this and invest money into the portfolio when it is relatively high:
- Case 2
- 1fund_badtiming.png (7.66 KiB) Viewed 178 times
- Case 2 - Value version
- 1fund_value_badtiming.png (8.66 KiB) Viewed 178 times
You can see in Case 2 the value version of the portfolio has performed worse.
The bottom line is that the overlaid value version suffers/benefits from bad/good market timing of external contributions when comparing relative performance of an index to a portfolio. The definition of good/bad here is good timing into the portfolio compared to timing into the investment. If they are both down when you invest, this won't make a difference, but if the portfolio is down more than the index when you invest, this helps the relative performance of the portfolio's value. I used a "linear" benchmark in these graphs, so the definition of good/bad timing is more clear by just looking at the timing into the portfolio.