The Google exercise card

One of the random things recent Android versions do is the Google exercise card, which at the start of the month displays the phone's guess at how many hours you spent walking and cycling last month.  I understand that it's not exactly a high-accuracy measurement.  They are a little vague, I guess based an an occasional sampling of the jostling as measured by the phone's accelerometers plus intermittent location fixes.  Anyway, um, yeah, not a high accuracy number.  The phone is always with me on the bike.  It thinks 6 hours in May and 7 hours in June.  The Garmin 910XT data is a whole lot better.  That totals up to 35 hours (and 706km) in May and 25 hours (and 577km) in June.  So in other words, the exercise card is basically worthless.  I don't log plain old walking, but I'm guessing the 5 hours isn't any more believable than the cycling number.