Hey dyene,
image you hold 1000 EUR in stocks which went to 1800 EUR. This is a performance of 80 % (steep green graph). Ate the same time you did have 100.000 EUR in a bond. This had a performance of 10 % (flat red graph), i. e. went to 110.000 EUR.
The overall performance is from 101.000 EUR to 118.00 EUR, equals to 17 % (blue graph) … you see not nearly close to the green graph.
You also see from that that converging is not a funtion of time but a function of the ratios of the securities.