Why is the difference between high school graduates and college graduates (0.92) significant, while the difference between postgraduate and less than high school (-1.29) is not?

Multiple Comparisons Dependent Variable: number of times prison is selected Bonferroni (D) LAST YEAR OF () LAST YEAR OF Mean SCHOOL RESPONDENT 95% Confidence Interval SCHOOL RESPONDENT Difference (1- COMPLETED COMPLETED J) Std. Error Sig. Lower Bound Upper Bound Less than high school High school graduate -.09672 .41001 1.000 -1.2519 1.0585 Some college .42125 43464 1.000 -.8034 1.6459 College graduate 81573 42635 .562 -.3856 2.0170 Postgraduate 1.28672 .47163 .066 -.0421 2.6156 High school graduate Less than high school .09672 .41001 1.000 -1.0585 1.2519 Some college .51797 27760 626 -.2642 1.3001 College graduate 91245 .26443 006 .1674 1.6575 Postgraduate 1.38343 .33253 .000 4465 2.3204 Some college Less than high school -.42125 43464 1.000 -1.6459 8034 High school graduate -.51797 .27760 .626 -1.3001 .2642 College graduate .39448 .30122 1.000 -.4542 1.2432 Postgraduate 86546 .36248 173 -.1558 1.8868 College graduate Less than high school -.81573 .42635 .562 -2.0170 3856 High school graduate -.91245 .26443 .006 -1.6575 -.1674 Some college -.39448 .30122 1.000 -1.2432 4542 Postgraduate 47099 .35249 1.000 -.5222 1.4642 Postgraduate Less than high school -1.28672 .47163 066 -2.6156 0421 High school graduate -1.38343 .33253 000 -2.3204 -.4465 Some college -.86546 .36248 173 -1.8868 1558 College graduate -.47099 .35249 1.000 -1.4642 .5222 *. The mean difference is significant at the 0.05 level