Age at immigration calculation error

USA
2

I recommend taking a look at this post as this issue has been brought up in the past. In short, YRSUSA1 in the ACS is derived from the year of immigration (YRIMMIG), which can easily cause a difference of 1 year depending on their age and when they responded to the ACS. The other cases are likely related to aggregation and binning the oldest years together, which are not reflected in the labels for YRSUSA1. In the 2019 ACS for example, everyone who entered the US in 1929 or earlier would have their year of entry set at 1929. You can see how these years of entry are binned by taking a look at US2019A_YOEP and the other USXXXXA_YOEP variables.