The great thing about market cap as an underlying ratio is that the balance automatically continues to add up. For example, if the emerging markets grow compared to the developed market, the underlying ETF will grow exactly in proportion. So when you stop investing, you do not have to actively rebalance the underlying shares, that happens automatically. Only the shares / bonds ratio will diverge.
When you combine accumulating ETFs, the different dividend percentage will cause deviation from the market cap, as paying dividends lowers the market cap. Because the dividend percentage will differ per ETF, the ETF that receives more dividend will increase slightly more than the market cap due to automatic reinvestment.