Hi, I‘ve had similar happen to me. As an example I went for an outdoor walk earlier today and according to the stats I walked from 12:57pm to 14:13pm so 1 hour and 16 minutes in total. The total time bit actually says I walked for 0:57:50, so completely different. My green exercise ring total says I have actually only done 45 minutes altogether so nowhere near the hour plus I actually did.
This used to irritate me as has happened many times, but when I look at the route map (know this can‘t happen on an indoor walk but same principle) there are areas of my walking route that are green, yellow and red. So I've surmised that some of the walk was green meaning I walked at a fast pace and it was clearly classed as ‘exercise’ for me, the yellow areas are at a slower pace so classed as partial exercise and the red bits are when you've stopped, taken a break or in my case pay for a drink in a store so ultimately stationary for a period of time.
I would say then that your indoor walk was at a slower pace (than the watches idea of exercise for you) so only part of the time you walked was classed as exercise.
When i do any exercise and want to close my green ring i keep going until it tells me it’s closed as can never tell based on time.
Hope that helps... Sorry for the long answer was the only way I could explain it!