Benchmark % gain accounts for recent purchases

When looking at the benchmark line versus the portfolio line after a new parcel of shares is purchased, the % return of the portfolio decreases as it is 'diluted' by the new parcel that has a low return. However, the benchmark line is unaffected by this. For it to be a realistic comparison of performance, the benchmark % return should also decline reflecting the value of a hypothetical parcel of holdings of the benchmark purchased at the same time, and the equivalent 'dilution'.