#1403 Statistics: update happens seemingly random

[Samuel Li]

To reproduce:
1) Open statistics, add a few variable. Note the statistics cells are empty.
2) Click "Update Statistics" button. All cells are filled with stats. (good)
3) Add a few more variables. Note that existing stats turn red (don't know why) and new variables have empty cells (good).
4) (I'm expecting to click the "update statistics" button to recalculate all statistics.)
5) Remove a variable. Statistics for all left variables suddenly turn black if they were red, and fill if they were empty. (bad, since I didn't click "update statistic" button.)
6) Add StdDev to the table, and all existing statistics update by themselves too. (bad, since I didn't click "update statistic" button.)