Basically, all of the combination locks had 2 digits of leeway. So if your first code was 20, you could set the lock to anything from 18 - 22 and it would still open.
This meant that instead of the locks having 100 x 100 x 100 = 1,000,000 combinations, they instead had something closer to 20 x 20 x 20 = 8000 combinations. He would then brute force the 8000 combinations through a combination of trying the more likely ones first and if that failed, just brute forcing it over a few hours.
The locks were made easier to brute force in that unlike modern locks, the 3 combinations would be input on 3 separate dials so you can set the first two to the 'right' value, then just spin the third while attempting the handle at the same time. This meant you could try literally every possible combination in ~10 hours of work.
As for an example of easy ones would be a birthday or anniversary.
- code 1 = day 1-30(6 combos)
- code 2 = month 1-12(3 combos)
- code 3 = year 1-99 (20 combos) (This would likely be 9-10 combos if he accounted for the distribution of likely dates)
So to try every date combo, you would only need to try 360 attempts in the worst case.