That is perfectly normal. The WEI was introduced with Vista and had a maximum score of 5.9 for each of the five categories tested/measured. With Windows 7 the scores went up to 7.9 max and with Windows 8 they went up to 9.9 max. In addition, refinements were made to the algorithms used to measure the performance of the individual components tested, and this can lead to the slight difference in WEI score between Windows 7 and Windows 8, as you have observed above.