I’m not sure if you meant it in the way that it reads but stocks and bonds absolutely do not return the same amount of money over time - why would anyone ever buy risky stocks if bonds returned the same? Also, if your 5% difference becomes that wild then you can try rebalancing every paycheck instead. There’s no downside to this other than needing to calculate more frequently. When you’re retired and no longer earning, you can sell from your portfolio’s overweighted portions to rebalance instead.
If one section of your portfolio has gotten smaller that means that part is doing bad or other parts are doing well. Buying portions that are doing bad means you’re buying them “on sale”. Buying portions that are doing well means you’re “paying extra”. The end result is similar to selling high and buying low, just like a sell+buy rebalance would, except that you’re only ever “buying low”. This changes to “selling high” in retirement.
To be more clear, you can sell and rebalance if you want but make sure you’re not causing taxable events by doing so - avoiding these taxable events is the primary reason to ‘only buy’ to rebalance (or ‘only sell’ in retirement).