Are tennis favorites winning less?

[mybookies]

I calculated the following win percents using data from tennis-data.co.uk

WTA
2010 - 71.7%
2011 - 71.0%
2012 - 70.4%
2013 - 70.8%
2014 - 68.8%
2015 - 68.1%
2016 - 68.0%
2017 - 66.5%
2018 - 67.9%

ATP
2010 - 69.8%
2011 - 71.3%
2012 - 71.0%
2013 - 68.6%
2014 - 70.2%
2015 - 70.7%
2016 - 69.3%
2017 - 66.7%
2018 - 66.6%

ATP Grand Slam
2010 - 79.8%
2011 - 79.7%
2012 - 78.8%
2013 - 78.9%
2014 - 76.3%
2015 - 79.1%
2016 - 81.3%
2017 - 77.3%
2018 - 77.7%

Notice the large drop off by favorites during the last 2 years. Has anybody calculated similar results or do you think the data or my math is off?

 

[musk]

how do you define favourites?