Why are intensity minutes erratic and not computed for automatic activity

Humm.. I know there are forum topics on intensity minutes and many questions have been addressed. I still have problems with erratic measurement though. I spent 35 minutes doing a walk/run session indoors on a flat surface yesterday and got a reasonable number of minutes. Today I went outdoors and did a similar session for a little longer and also ran uphill (19 flights in total) and got half the intensity minutes. Truly bizarre.. I have another question as well: if you don't explicitly start an activity, you get NO intensity minutes for the same type and length of exertion. Even with 'auto recognized' activities, you get no intensity minutes.. also bizarre. Maybe they should do away with them and just count active calories. At least that seems consistent even if the 'projections' are somewhat random. Anyone have any insight?