Gender ratios in physics students in the US and UK....

Why do A-level physics gender ratios in the UK and BSc physics major gender ratios in the USA show similar trends offset by 15 years....?

In the UK the trend flatlined (or began declining) around 1985. I the USA the flat line/decline starts around 2000. There should be an offset of about 4 years (the length of time it takes to get a degree in the USA), but I don't understand 15 years.... 

Modelling Data - Example using A-level Physics Gender Trends

Love this blog post: "Brent and Levenberg-Marquardt: the bread and butter algorithms for postgrads" which uses data I collected as an example for modelling trends.

I collected these data to include in an article I was invited to write for Astronomy&Geophysics on "Women of the future in the RAS". In that article I conclude: "Fitting a straight line to this 60-year trend and dangerously extrapolating the poor linear fit into the future, we find that we can't expect gender equality in physics A-level until 2163."

The data is not cheerful, and a linear increase model does not fit it well. In fact according Val Aslanyan's best fit model, recent years show a decline in the fraction of A-level physicsists who are women. 

Credit to Val Aslanyan (http://improbablematter.blogspot.com/2016/12/brent-and-levenberg-marquardt-bread-and.html) for this version of this plot.