Today I bought 35 [Fleet Tactical Personnel - Common Quality Duty Officer] 6 times and I got significantly more Tactical DOffs than Security.

Here are the results from each trial.
Security-------Tactical
2---------------33
4-------------- 31
6-------------- 29
1---------------34
5---------------30
7---------------28

If my main goal was to get security DOffs from these boxes I would have spent 157,500 Fleet Credits to get 25 of the DOffs I was looking for and I would have had to figure out what to do with 185 Tactical DOFFs.