I stayed in the US for one year, left the US and was readmitted a few months later. If I overstayed, why did Immigration allow me back in?

Applying for adjustment of status but came across the issue that I may have overstayed the I-94 (which I lost back in 1998) because I left the US a year later on my 10 year visa. Supposedly the I-94 would not have a year expiration date so its likely that I did overstay, but I left the US (which would trigger the bar) and returned a few months later without any issues. Maybe the I-94 was valid for a year?