What you are describing JW is energy compensation mode. The jet is burning energy by going for the high speed but it drives the bug to flap speed. If you leave it without interference as the energy is burned in descent the CAB will return to the descent speed as long as the speed remains 5 knots below Vmo or 255 knots below 10,000 feet.
The jet has two descent modes, performance and geometric. Performance will hold height until a thrust closed descent profile is obtained. Geometric draws a line between two points an will average the descent rate between the two points.
Not all B737 have geometric path enabled.