What you're doing is essentially a technique called Greasing the Groove. There have been a number of questions on here already about it, but essentially it's a method of becoming stronger in a particular movement pattern by practicing it constantly throughout the day.
Note the use of the word "practice" there. The idea is to do exactly what you're doing, about half your max reps at intervals throughout the day.
The theory, as I understand it, is that by repeating a movement pattern with good reps (which they all should be if you're only at 50% of your rep max), you're teaching your body to become more efficient at that particular movement pattern.
Stick with it for a few weeks, then test your rep max again, see if you've improved. If you have, you know it's working.
(On a side note, I'm totally jealous of you having a pull up bar at work, I've repeatedly asked for one in the office, and my boss keeps looking at me like I'm insane...)