z-scores and scatterplots over time

Bruno Martorano (UNICEF/Innocenti) presented a paper at the ICSI Conference titled Child Well-being in Economically Rich Countries.
Many of the figures employ a simple scatterplot with report card 7 on x axis and report card 11 on the y axis. The scores are standardized and represent distance from the mean in sd units. I have never seen this strategy for showing change over time. Curious what caused Canada to be almost 2 sds below mean for report card 11 (p. 15).

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