indeed an interesting observation. the Austrian econ gang is always using Japan as the bad example. how on earth is their currency doing better than all other?
It's worth noting that the gap isn't fully explained by the exchange rate. Prices in JPY were running a small but consistent premium over USD.
The rough table below uses the top 10 days for JPY during 2017/18 bull run and converts to USD equivalent using the exchange rate at XE.com. Prices work out 2.5% - 6.7% above Stamp.
Date JPY XE USDEQV USD %DIF
1 17/12/2017 2208232 0.008880 19609 19110 2.6%
2 18/12/2017 2147476 0.008896 19103 18634 2.5%
3 19/12/2017 2138407 0.008849 18923 18430 2.7%
4 16/12/2017 2112793 0.008881 18764 17986 4.3%
5 15/12/2017 2002191 0.008874 17768 17334 2.5%
6 20/12/2017 1992847 0.008826 17588 16854 4.4%
7 06/01/2018 1987945 0.008847 17587 16724 5.2%
8 07/01/2018 1981675 0.008847 17532 16670 5.2%
9 12/12/2017 1937334 0.008993 17422 16434 6.0%
10 21/12/2017 1935878 0.009036 17493 16397 6.7%