Do you have any evidence that they didn't know how to measure out miles in the 1800's or is this purely speculation on your part?
JSS says the reason is underwater mountains. Now you are claiming that they were too dumb to measure things. Any actual evidence for any of that, or are your own wild statements enough?
There is plenty of evidence that yes, in the 1850's they had a hard time measuring some things, like the bottom of the ocean.
I explained this before. Until sonar was developed, the only way they had to measure the depth of the ocean was to literally drop a rope into it and try and feel when it went slack.
This is a very, very, very slow process. It's also highly inaccurate. They can only do spot checks and try and extrapolate from a small number of vague data points. They also had no way to accurately measure their position as wind and currents took them off course and they had to correct, so the path was not a straight line.
So yes that is exactly what I am saying, in 1855 they did NOT know how to measure an undersea path because they could only guess at the true shape. I will repeat...
From your Wiki: "Total amount of cable paid out, 949 miles; total amount run by observation, 818 miles; ... Surplus cable paid out over distance run by observation, 131 miles "
All it is is saying they measured 818 miles traveled over sea and dropped 949 miles of cable. Exactly what one would expect in the 1850's when dropping cable over unknown, uneven terrain while only being able to roughly track their surface position as they drifted.