Unusual speed problem

While attempting to test my homemade STAR, I encountered a speed problem. My cruise altitude for the trip was FL200 (short trip). Upon reaching T/D, the aircraft slowed to 240kts, and descended to FL187. VNAV and LNAV were both engaged. On the descent page, I had 290/.755 chosen as my descent speed, with the usual 10,000 foot restriction of 240kts. On the speed tape, the speed bug was down at 240kts. My climb, cruise, and descent speeds were all identical, 290/.755.Any ideas why this happened? My STAR is nothing out of the ordinary. No speed or altitude restrictions, just a windy path consisting of six legs.Kenny Rogers