Picture yourself travelling in a straight line.

Put a dot where you are every second.

Add some innacuracy to move the dot in a random direction by a small margin.

If it goes straight forward or back, it comes out in the wash, it looks like one reading is quicker and the next is slower, but if the dot goes off the line you are travelling in (it will) then it will ALWAYS result in an average increase in distance travelled, therefore makingyour speed look higher than it is.