There's been some great responses to this post so far, and I'm glad I posted it. It all makes sense, and I'm going to try fumbling around with my Garmin to see if a change of settings will produce more consistent results.
For the record though; I CRUSHED, CRUSHED ,CRUSHED my segment today!!!! Although the Garmin read 1 minute 35.85 seconds, I got credited for 1 minute 39 seconds which took an entire two seconds off my previous time of 1 minute, 41 seconds which I've been trying to beat for three years!! Not only that...I am now in the top ten (10th) for my age group for that segment!!! My ego is through the roof right now, and I probably need bigger shirts and hats as a result.