The information on the 1900 census form seems redundant: year of immigration to the United States and number of years in the United States. After all, shouldn’t the knowledge of one allow us to infer the other? Couldn’t we just subtract the year of immigration from 1900 and arrive at how many years the person has lived in the United States?
If an immigrant came to the the United States, returned to the home country for a few years, the then re-entered the United States, the information contained in these two columns is not repetitive.
Do you always check those columns on the 1900 census for consistency?