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....