If a competitor scores or finishes "dead last", that generally implies that the competitor's performance was much worse than anyone else's. For example, if competitors finish with times of 3:13.2, 3:13.9, 3:14.2, and 5:39.7, the difference between first and third would be only a second, but the difference between third and fourth would be more than a minute and a half.
There isn't an exact formula to determine when the phrase "dead last" is appropriate; it tends to suggest that the difference between the competitor's performance and the next better competitor is large in both absolute and relative terms. It's possible for multiple competitors to finish "dead last" if the differences among their performances are small compared to the difference between the best of their performances and the performance of the next better competitor (e.g. in a 6-contestant race, if competitors finish with times of 1:00, 1:01, 1:02, 1:03, 5:37, and 7:23, the last two contestants could be described as finishing "dead last", since the field may be divided into "people who finished in under 1:05" and "people who took more than five times that long". The latter two contestants might officially be credited as "finishing", but their performance would be qualitatively worse than anyone else's. Had the sixth competitor's time been bad enough, it would make sense to describe that competitor alone as being "dead last", but there's no clear line where the distinction should be made.