This is not how it works. The weighting is based on the Consumer Expenditure Survey. BLS does not arbitrarily assign the weights. If they change the weighting between butter and bread it's because they found that people were buying more bread in the CES not because they assume that's what will happen.

CPI's methodology is transparent and the data is available if you wish to reproduce it. They aren't playing games with the data. There are all kinds of reasons your personal inflation rate might differ from CPI but it's not because BLS is putting their thumb on the scale to try and show less inflation.