I believe that it's because the 1931, 1932, 1934 and 1936 all had relatively low mintage that there is not a great price difference. But sometimes coin prices can be difficult and confusing. Look at the different prices between 1921 and 1929, just a little more than 20,000 (1921: 206,398; 1929: 228,328)
and there is a HUGE difference in price!
and there is a HUGE difference in price!



















