Nope, it's just jittery data. (I just did the same and got the same figure as you in Excel)
Take a look at lines 866 to 868:-
866 52.81203 -3.61643 81.431 392 -20 0
867 52.81223 -3.61678 81.464 421 29 29
868 52.81305 -3.61744 81.565 405 -16 0
Now have a look at:
52.81203,-3.61643 to 52.81305,-3.61744
in google maps and click on the Terrain button.
Does it really drop 20 meters, climb 29 meters then drop 16 meters in that short distance? Nope.