As far as I understand, GC works exactly as you described - you have to be quite precise in distance to set activity as PR.
To match PR's on GC, you have to match distances closely.
My current 5K personal record was set from an activity with a total distance of 7.58km. (The activity that set the personal record is linked in the personal records table, so there is absolutely no doubt or mistake.)
People are confusing Garmin Connect personal record calculation and the more sophisticated behaviour of some (but not all!) Garmin devices.I don't think we're confusing anything. I, for one, was talking about my personal records as reported in Garmin Connect, and from my reading, the fellow forum member who started this thread was asking about his/her personal records as reported in Garmin Connect.