The 'distance walked per day' graph's axis scale labels do not seem to tie up with the actual distance walked (unless you're just starting a day and have walked ~0.0 miles so far).
In the below image, on Monday, I walked 7.3 miles so far on that day, but the graph below it shows well under 6.6. The size of the bars also roughly tie up with the number of steps walked (the page above), but the distance scale is represented incorrectly on this page. On Saturday I'd also walked 9.8 miles -- that's a long way off what's represented.
This may have something to do with custom stride size being counted in the numerical measurement but being taken account on the graph, or some other multiplication not being done quite right. I am over 6 foot tall and as such have quite a long stride distance, so I'm guessing that may be why I have noticed.