The difference in the calculated speeds for the Pacific plate for the two different time periods could be due to changes in the rate of plate motion over time. Plate motion can be influenced by a variety of factors such as changes in mantle convection currents, the geometry of the tectonic plates, and the presence of obstacles or subduction zones. Additionally, variations in data quality and measurement accuracy could also contribute to differences in the calculated speeds for different time periods. Therefore, it is important to take into account the specific methods used to calculate plate motion and the potential sources of error in interpreting the results.