It's a bit sneaky to say it generates all three-digit numbers when 998 clearly isn't there, but as onedognight pointed out below, the leading 1 from the 1000 wh

It's a bit sneaky to say it generates all three-digit numbers when 998 clearly isn't there, but as onedognight pointed out below, the leading 1 from the 1000 which follows 999 will cause 999 to overflow into 1000, which turns the 998 into a 999.